Hello all, hope someone may assist me in resolving this issue. I have a 2003 Chevy s10 4x4.
When I press the 4 hi button it stays on,it supposably locked In 4 WD. I hear a whissing noise and slight clunk, same in 4 lo, and it does go into 4 lo. Problem: the front wheels do not have power to them in either 4 hi or 4 lo. I have replaced the the actuator diaphragm under battery tray, and the transfer case vacuum switch on top of transfer case, has 3 rubber vacuum lines attached to it. These part replacements had no effect on the power loss to front wheels.
Thank you for your help. Bob

 

Sounds like you have a vacuum leak your 4wd is vacume actuated I have the same problem on my 1999 s10 its a matter of finding the leak it could be anywhere