4x4 issue

I have a 96 gmc k1500 4x4, and I have a bad clunking/ rattling sound, ONLY when in 4x4. 4x4 drive works though, and this sound is on all types of surfaces. I also have bad wobble and vibration 60mph and higher even when in 2wd. Another issue I have is getting into 4w hi and then back to 2wd. Only way to get it back into 2wd is to put the truck in park, the transfer case in neutral then go to 2wd. I'm thinking that's linkage or something, but my main concern is the bad sound in 4x4.