why does my 1997 gmc jimmy make a loud clunk in noise?

[Rick J]

when i shift from 2 by to 4 by and drive a short distance a loud noise comes from the front of my 1997 jimmy the same thing happens when i shift from 4 by to 2 by

 

[Non Compos Mentis]

Check the drive shafts and input/output shafts going to transfer case and on transmission.Would also check that transfer and front and rear ends are full of gear oil.

You can get under vehicle after putting chocks no wheels and setting parking brake and have some one switch back and forth to hear where noise is (truck running) or put it on a lift which is much safer.