Advice needed - AOD clunking? or motor mounts and u-joints?

Hello everybody. Could use some advice. My 90 GT 5.0 with an AOD, While I'm drivingand just as I let off the gas at any given speed somehwere in the driveline there is a semi loud clunking. Sometimes happens when I give the car gas as well. It's a pretty dramatic clunk, enough that you can feel it in the pedal, and in the seat. My motor mounts are a bit cracked but by no means broken and I visually inspected the u joints and they apear to be fine. Could this be my throttle linkage or shift linkage? Advice apreciated.

On another half related topic; I attempted to change my motor mounts this last weekend using some anchor brand mounts from Autozone. I get them on the car, bolted to the motor and only then do I realize that the threads on the main stud which goes throught he frame are metric and my peened stock bolt is standard. I was furious and had to put the stock mounts back on. It was the 4th so there were no stores open to get fasteners.
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Sometimes when a ujoint goes out you can't tell by looking at it unless you remove the driveshaft. I would start there and if that's not it, have a transmission shop drive it, it could also be the torque convertor in the tranny.
My chevy used to make a clunking noise and idle to near idle rpms. One day car wouldnt start, engine wasnt turning over but the flywheel was turning. Found out that the flywheel broke away from the engine. You might have to look into that.