98 and newer will NOT fit. That is the "modified" 12-bolt...pretty much everything is different except the housing, including the gear ratio. Everything 97 and older should fit and have the same 4.56 gears...some might have an ABS sensor ring (for rear ABS) but I don't think that is a problem for you.

And yes, you can swap out the "third member" (don't ask me where the name came from) without removing the axle. You do have to pull the axle shafts out a few inches to disengage them from the diff, then it is a matter of unbolting the driveshaft and the third member housing and pull 'er out.

It is not too difficult, I have done it a couple of times now. The hardest part was re-assembling the emergency brakes the first time...but your e-brake might be different than mine. I'm a little hazy on first gen details.