I think it's my transfer case

Went on a trail ride with my club this past weekend and have developed a new problem. The problem is when I accelerate in first gear from a stop I get a clunking sound and feel it through the floor boards, when I'm in 4 high it gets even worse, but I get nothing in 4 Low. If I accelerate slowly I get nothing. All rear suspension componets seem tight and in order except for the rear driveshaft u-joint (at the pinion) it's a little loose but not bad. It's not my yokes binding either. Any help would be great.
'92 XJ 4.0 5spd NP 231 3" lift.

Thanks

Forgot to add I swamped it and sat there for about 10 min. while I got towed out. Water in the Tranny and diff's but not the transfer case. I changed all fluids.

Thanks again

check to see if the exhaust pipe is hitting the xmember

I would check motor mounts and tranny mount.

Thanks for the replys It's not the exhaust and it's not the mounts all looks okay and tight.

Thanks