i have a 99 automatic 4x4. i havent had any issues with 4 wheel the other day. i put it in 4 high went in fine and was fine for a minute then it suddenly clunk and truck hopped stoped and then a few seconds later did it again. i checked all u joints. could this be a bad transfer case?

 

If you manually hold it all the way back in 4hi does it work ok? It is common for the shift fork to wear out and cause it to not fully engage into high range. When mine did this, 4 lo worked fine. 2hi and 4hi both kept kicking to neutral. I replaced the shift fork and input shaft to repair it.