I think our original poster friend is talking about the VG150 transfer case, of which I am completely ignorant. But since the thread has turned bit toward the VG080, I thought I'd point out that there are two distinctly different varieties, and I think maybe another variant on those, though I'm not sure of that last part.

Here are a few significant differences:

1- the left side cover and mounting - an early type of VG080 uses a left side cover that has the mounting points almost one dierctly above the other and then has a steel bracket holding the rubber mounts, and the steel bracket then bolts directly to a mounting plate on the chassis.  A later variant of the VG080 has the mounting points on the left side cover staggered more front-to-rear and those points bolt directly to the rubber mouynts held in flanges welded to the chassis (no steel bracket).  The good thing here is that the covers are interchangeable so if you get a t-case that's different from the one in your truck all you have to do is use the left side cover from your old t-case on your new one to get the new t-case to fit your truck.  The right side mountings are all the same.

2 - Shaft thread/spline diameters - I'm not sure if any of the internal shaft dimensions are different or not, but the splined and threaded portions of the shafts where the flanges mount are different in different vintages of VG080.  Older ones had smaller diameter threaded portions and different splines, and smaller nuts holding the flanges in place, where newer boxes have larger splined, threads, and nuts.  So it follows that the flanges do not interchange from older to newer boxes and vice versa, even though all of these flanges have the same 8-bolt pattern for the drive shafts (I HEAR there were some very early t-cases with 6-bolt flanges, but have never seen one).

3 - Oil pump assembly method - Older VG-080s had a crush-sleeve on the rear output shaft to pre-load the inner (driving) sleeve of the little oil pump that's in there.  This must have proven unreliable beacuse although the crush sleeve is easier to assemble with proper preload, MB later switched to shimming as a method of establishing preload on the output shaft.  It takes a little more care to set up this later style, but I think it's more reliable than the crush sleeve.  The machinist in me never could really trust those.

I'm not sure if all of these changes happened together or if they were staggered at all, but of the four transfer cases that have passed through my hands for various reasons, there were only two varieties, one that had all the "early" atributes, and one that had all the "late" attributes.

Long story short is, make sure the workshop manual you have matches the transfer case in your truck before you hand the lot over to your mechanic.  Or at least be aware that you might need to get more info once things are apart.  I have manuals for both types of t-case if you get in a bind and need some info you don't have.  I even have some spare parts I can't use due to ordering for one type in advance of finding out the box in hand was actually different.

Also be aware that some parts for the early style t-case are no longer available.  That's what ultimately drove me to look for a later style box when portions of my OE early type box gave up the ghost.

-Dave G.