I have a '90 S10 Blazer...when I put it in 4WD, I hear a high pitched spinning sound when I accelerate, and it goes away when I slow down. It sounds like it's coming from the engine area, but it's hard to tell.

The sound goes away when it's back in 2WD. I'm not very experienced with mechanical issues so hopefully somebody can help me out! Thanks!

 

some older 4x4s will do that, if its like a spinning gear sound id like to say its normal as ive owned a few older trucks and even my 93 s10 does that, especially in 4low due to the gear reduction. check your front differential fluid and make sure the front driveshaft isnt rubbing on anything. when its 2wd the front driveshaft isnt spinning