1998 Chevy Silverado loud clunk when shifting to 4hi?

For the last year when I shfting to 4hi I have had to slow to about 5mph or it sounds and feels like something is going to break? 4 low also is loud shifting even at dead stop which it has to be to switch into 4 low. It still has always shifted into 4x4
but I fear it will fail me this winter. All the local mechanic say as long as it goes into 4x4, drive it due to the high mileage, I due almost all my own repairs but not sure where to start transfer case? Just looking for someone else that has same problem and how to fix it.
This is a push button 4x4