Replied, best of luck!

Since the subframe bushings are the most difficult to replace of the several possibilities for the clunk, I would start by checking the rear calipers and rear suspension components. This video will help, wait until he gets past the transmission stuff. https://www.youtube.com/watch?v=HkpuYnaeWUg

You can usually see cracks in the rubber around the bolts if the giubo is bad. This video will give you an idea of the scope of the job. https://www.youtube.com/watch?v=4yCxhyTlysw

It is difficult to diagnose if the problem is the subframe bushings. Usually if the subframe bushing are bad you will hear a cluck when you go over some like a speed bump as well as when you move forward or back. I've attached a video on replacing subframe bushing. I recommended bending the muffler bracket out of the way rather than cutting it off.
https://www.youtube.com/watch?v=haKecoxhWsk

I get no clunk when going over a bump or many bumps, in fact...

The only clunk is on decelleration, and only at low speeds... I had my mechanic check it out breifly, the clunk is trasmitted right thru to the trailer hitch, we can even feel it when touching the trailer hitch... (I went forward and back slowly in the shop)

Noise is definately rt rear...

sure its not a heat shield or something? clunk in rear on just one side...interesting

Since the trailer hitch is connected to the frame the vibration will travel along the frame. The closer the problem is to the frame the more likely you would be able to feel it at the trailer hitch.

I would check the upper then the lower suspension components for bad bushings, ball joints, shocks, sway bar links etc. That would include the sub frame bushings even though there are indications that is not the problem.

I would get it up on a lift and see if it will clunk that way. Probably won't because you won't have load when you back off the gas. Braking might make it happen. Mechanic didn't have any idea where the clunk was originating? Could be the rear diff also. So many options, need to keep troubleshooting to narrow it done. Not unusual that when you can hear a clunk you can visually find it without making it happen. Spare is locked down I assume.

tried on lift.. All bushings etc were checked by hand AND with XL pry bar.... Couldn't dupplicate noise while on lift....Suspension needs to be loadedd to duplicate... also, took spare tire and jack out to test.... clunk still there....

I'll keep poking at it

Your are doing the right troubleshooting rather than buying parts to find the core problem. Keep posting.