does it feel this way throughout every gear, as if the clutch was slipping perhaps?
it could also have a non-welded center diff, and have either a broken axle, transfer case, or issues within the viscous coupling. all of these issues would happen with a regular 2-spider differential, as it would simply try to spin the broken axle/tcase/etc (path of least resistance) also, thick silver goo in the transmission oil would indicate a bad viscous coupling.
id say to check your axles, driveshaft, and transfer case for proper operation, if all of these are ok, then go through the transmission. when you get the center differential out, take the top 8 14mm headed bolts out, remove the top of the center differential, and look at the insides. there will either be two spider gears (or 4 if you're really lucky) plus the top and bottom gears, or if it is welded, no spider gears and the top and bottom gears will be welded to the housing.