1995 Grand Cherokee drivetrain problems

Hey everyone, hope somebody knows the answer to this one. I have a 1995
Jeep Grand Cherokee Limited with a 5.2L V8, full-time 4wd. It has an
automatic with the NP249 transfer case. When I drive it, especially at
70mph or more for a mile or so, the Jeep starts making a thumping sound
in the driveline and something is trying to bind up because I can hear
the tires faintly screeching on the asphalt. The first time it
happened, I thought I had a flat. It was that kind of sensation. I can
pull to a stop, then take right back off and things are fine until my
speed comes up, then all over again. I can hear it affecting my
driveline, sounds like somebody pecking on my driveshaft with a small
hammer. Here is what I have replaced so far:

Rear driveshaft U-joints
Front driveshaft from end to end
Front axles
Front spider gears

None of these have helped which has led me to believe it is the
transmission or transfer case. Anybody have any ideas?

Thanks,
Derrick