The atlas II transfer case by Advanced Adapters has several advantages over the stock NP231 or NVG231

• Superior strength
• Shorter (6½" shorter than an NV231 without a short-shaft conversion kit) allowing for better drive shaft angle on lifted jeeps
• Shift-on-the-fly feature
• Low-range ratios of 3.0:1, 3.8:1, 4.3:1, 5:1 or 6:1 are available
• Individual axle control with two shift levers so you can have (1) front wheel drive only (2) 2WD low to reduce drivetrain binding when one encounters tight turns on the trail with lockers.

Operating Instructions (Pdf)
FAQ's

Oil:

If you are unable to find these lubricants, one of the following synthetic oils can be substituted: Castrol Syntec 5w-50, Valvoline 20w-50 or Mobil 1 Synthetic 15w-50.
Some say you can also use Redline MTL 70-80

GL-5 rated fluid, such as Amsoil SEVERE GEAR® Synthetic Extreme Pressure (EP) Lubricant, is corrosive to the synchronizers.
The corrosion wont happen overnight but in the long run will damage the components & cause shifting issues, which will void the warranty.

Replace oil by removing upper connection for view tube.
For topping off the Advanced Adapters hotline said, you can remove the speedometer gear. Alan bolt holding U ring. However, when we tried this oil came out.

It has a tendency to loose oil thru the vent tube/breather. Too much fluid will cause foaming at high speeds. Foaming oil expands and fills the entire cavity, which forces fluid out the tube; or the breather hose may be restrictive, causing the Atlas to build up pressure. On late model Jeeps, we have found that you must replace the restrictive breather (located on the firewall) with a free flowing atmospheric vent and larger tube.
The tube should not have any dips.

Excerpts of installation notes:

4x4now.com/butjc.htm
The Rubicon Express 4 1/2" lift control arms then had to be adjusted to align the differentials with the new drive shaft angles. Since both were now CV shafts, the pinions needed to be nearly straight with the driveshafts. The rear control arms had to be adjusted significantly to point the Dana 44 up at nearly the same angle as the new 19 1/4" CV driveshaft. This took some effort since the rear arms need to be unbolted and freed on one end to make the adjustment.

The test drive did reveal the need for a little tweaking though. There was a the thud of metal hitting metal when I hit a good bump and when I accelerated hard. The transfer case was bouncing up (or rotating up in the case of hard acceleration) to hit the stock shifter. It took only a few washers to space down the skid plate just enough to avoid this problem. It's a tight fit for the massive Atlas II inside a TJ.

www.fourwheeler.com/projectbuild/129_0309_1997_jeep_wrangler_tj_atlas_transfer_case/

A minimum of 1/2 inch is recommended to keep the body from knocking the shifters, and we had to do some cutting of the floor to achieve that.

When we installed the skidplate/crossmember combo we found that the Atlas smacked the crossmember. We didn't want to lose valuable clearance by using spacers on the crossmember, or by notching it, so we decided to take the Atlas back out and re-index it one position higher. Once this was accomplished, it fit with no problem. However, our TJ has a 1-inch body lift, so if yours doesn't, some banging on the floor with a hammer for clearance might be required.

In the first installment of the TJ project, we installed a Tuffy center console. Now we had to do a bit of trimming on it so that it would clear the shifters.

rockcrawler.com/techreports/aa_atlas_tj/part2/

The modifications that need to be made to the crossmember are much more involved. The installer should have sufficient metal-working skills to ensure proper fitment of the Atlas II transfer case. Steve took measurements and cut a gap in the boxed section of the crossmember to allow clearance for the left-hand drop portion of the installed transfer case. After cutting the cap, he started work on the forward portion of the crossmember that allows the transmission mount to seat correctly. In this case, he had to cut around the front three sides of the forward mounting slots that are used to provide a mounting location for stock drivetrains with automatic transmissions. Once three sides were cut, he used a hammer to bend it up at an angle to match the original mounting position. Steve then welded the forward mounting surface into its new position.

As is, the movement of the Atlas II shift levers was extremely limited, so he had to cut a relatively small section of metal away from the front of the hole in the floor. Without doing so, the Atlas II wouldn't engage in low range because the levers were obstructed.

Once the hole was large enough for the shift levers to move completely unobstructed with the center console installed, Steve cut a perfect oval out of a square piece of sheet metal. He placed the the sheetmetal over the enlarged hole, and riveted it to the floor. He then painted the sheet metal addition, and mounted the boot around the shift levers. Be sure to grease the boot and invert it before reinstalling the center console, and be sure the levers don't bind up on the boot before final assembly. Even though I summed up the lever installation process in about a half-dozen sentences, don't be fooled! This is possibly the most difficult part of the installation, depending on your vehicle. It took Steve several frustrating attempts to get it "just right".

Overall, I am pleased with the installation. It took about twelve hours from start to finish at a healthy pace. If you aren't doing it yourself, expect the shop to have your vehicle for at least two days.

From Operating Instructions:
The Atlas has internal shifter interlocks that prevent the unit from being mis-shifted. 1. Test 2WD Hi first.< 2. Shift the "Front" shifter knob into High, putting you in 4WD High. (On manual locking hubs, to achieve any type of 4WD, your hubs must be locked) IF THE SHIFTER DOES NOT ENGAGE EASILY, DO NOT FORCE IT. (Your linkage my need to be adjusted). 3. While the vehicle is slowly moving forward (< 5 MPH), depress the clutch and engage the front or rear control into Low. DO NOT FORCE THE ATLAS INTO GEAR or HOLD PRESSURE ON THE SHIFT LEVERS WHILE RELEASING THE CLUTCH.

With the front in 2WD low, the torque is distributed 100% to the front axle. This factor, combined with the low gearing in the transfer case, can cause undue strain on your front axles. This should only be used for quick tight turns in loose soil conditions with Positraction equipped front differentials.

Links:
atlas-tc.com/instructions/atlas.pdf Transfer Case Upgrade
Atlas II at: 4x4now and RockCrawler.com
Advance Adapters (Atlas mfg.)