I didn't either, apparently its the piece on the front driveline that has 2 u-joints stacked next to each other where the driveline attaches to T-case.
Sounds like a constant velocity joint on a driveshaft. They look kind of like an H but I've never heard them called an H-block. If that's the problem it should be pretty cheap to fix.
If the viscous coupling is bad that can get expensive to replace but I think you can still drive them even when they're bad. I know you can when they go bad in the AMC Eagles I have & the couplings in them are the same as the ones used in earlier Jeeps. I've actulally swapped an NP229 Grand Wagoneer case into my Eagle so it has a low range.
I think you stole that Jeep for $500. Core charge on a transfer case is $300.