grinding sound and klunk on deceleration

When my 93 Jimmy slows down (after going over 80Km the car occasionally wines as it slows and then I hear a klunk under the car and everything is fine. The problem is getting worse but the mechanics had the transfer case apart and says there is nothing wrong with it. If I have it in 4W HI this problem goes away.
Please help me its been to 3 different mechanics and parts experts with no luck
Brent

It sounds like your front differential is partially engaging. The most common cause is the vacuum switch on the transfer case. This vacuum switch controls the vacuum to an actuator located under the battery tray. The actuator controls a cable that goes to the front end to engage and disengage the front diff. When the switch goes out it won't let the actuator work properly, you may even notice tranny fluid in your vacuum ball and/or A/C contol vacuum lines. The switch is less than $20, but you should also inspect the vacuum lines, actuator, and cable for proper operation.

If you go too long without fixing it, you can destroy the gears in your front end which would be a very expensive repair.