Tranny Problem/ Transfer Case?

I have a 97 Ford Explorer Sport that is in its 4th visit to 2 local shops in 4 months time. We started feeling (almost in the driver floorboard?) & hearing a thump in the rear end after hitting bumps in the road. Sorta sounds like you're crossing the dotted white line in the middle of the highway and you hit one of those plastic reflectors- that's the noise- except you aren't hitting a reflector. It's coming after a bump/ railroad tracks/ and now when going uphill, accelerating...used to be we would be going 60mph to make it happen, now it's happening at 25. Aamco transmission shop has no clue how to fix it. They thought it was either the rear wheel bearings and seals (which we replaced), the rear emergency brake shoes (which we replaced) or the differential needed draining. (which we paid to happen) Still the same problem. Another local shop has it now- told me almost 3 weeks ago that he thought a gear or chain was stripped in the transfer case- except he puts the truck on the rack, takes the xfer case apart, and finds that it's in good shape. Doesn't need to be replaced. He delves a little deeper, and finds a wave spring broken off in 3 pieces, wedged in between 2 other parts inside the xfer case. Paid $100+ to get the part, they install it. Put everything back together- and the thump in the rear end is still there. Today I am being told it might be the "parking pall" and they have pulled out the transmission pan. Can anybody help us with this??? The transmission was overhauled/ worked on by our local Ford Dealership over 3 years ago. The truck has 142,000 miles on it- 1997 year model. The noise/thump is getting worse- the mechanic has also noticed that he put the truck in reverse, it wanted to stay in neutral, he gave it a little gas, and the truck "jerked" and went into reverse- it's done that a couple of times with us- and the mechanic has noticed it in the 3 weeks they've had it. They have taken the truck for many test drives..

sounds like a problem with either the transmission mounts or a tourqe converter lockup problem sorry i can't offer more help i feel your pain though. get a new tranny. how are your rear springs could it be axle wrap?

Thank you for responding- I would have to ask the repair shop who has the truck, how the rear springs are and if it could be axle wrap. I have no clue! But you've given me another avenue to explore-

Thank you again!

Sean & Bettina Jahr