Strange question but no one knows the answer.. even the guys at the dealership. I have a 1997 S-10 4wheel drive 5 speed. When I shift into 4th and always into 5th gear the 4 wheel drive unit will try to engage.. usually when I hit better then 50 mph. U can hear the whinning of it as U drive. It will disengage when U slow down to about 3 mph and go CLUNK or U have to shift into 4 high then back to 2 high. When U engage the clutch the whinning is very loud.. when U release the clutch and hit the gas the sounds is barely audible. Anyone have a clue here? BTW if U saw the movie Crocidile Dundee II when he was calling friends from on top the mountain twirling that thing thats what it sounds like.
Guess is vacuum leak around the t-case switch. Try disconnecting the vacuum line to the front diff and pluggin it. Run it that way and see if it stops. If it does, your t-case vacuum switch is leaking.
IF you notice when you hit a bump it might select 4 lo or hi. While parked take out the switch (from Dash) put truck in gear with the brake on and tap the switch watch it switch. If that is the answer replace it form the dealer.
You also notice the light being dim on the old and bright on the new. Two modular clips. This should be the problem. Cost me $75.