as the title says, i have an 04 silverado 1500 4x4 that will drive fine for about 15 to 30 seconds, then it will act like it's in neutral. Doesn't matter if it is in 2wd, 4 low, or 4 high. If i shut it off for about 10-30 seconds , i can fire it back up and it will run completely fine. I just replaced the transfer case on it and the tccm.I also have tried 2 different encoder motors. any ideas?-

Also, when it's in this "neutral" state, it won't shift from 2wd to 4wd or anything.

 

I'd try checking all the relays. That MIGHT be your issue. After 16 years things tend to deteriorate on any vehicle..

 

Check for codes even if the CEL isn’t lit.

Are you noticing if the 4wd indicator lights OR the gear select display in the cluster is flashing when it kicks into neutral?