Clunk Diagnosis Help..., Thought it was fixed, but noooo, couldn't be that easy!! Options

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1st, a little backstory...
I had a fellow teener drive my 75 2.0 a few weeks back, and he commented that my gear lever was moving waaaay too much when he accelerated/decelerated, like my motor mounts were bad. I used the good Dr's motor mount test method (try lifting the engine with a jack while mount is all bolted up) and found that it wasn't the motor mounts (they held the engine down to the mounting bar just fine) but instead was the transmission mounts that were causing the issue, as the main nuts were barely finger tight, allowing the transmission to move around quite a bit. I tightened them up, and added lockwashers to prevent this from happening again...

BUT NOW, I have a very solid, and quite concerning clunk coming from the passenger rear of the car, ESPECIALLY when I accelerate and decelerate. It seems that the changeing load on the engine/transmission is moving something, but I cant figure out what???!! It really doesn't make any noise when hitting bumps, or changing suspension loading (corners, sudden stops, etc.) ONLY when I push the gas, or let off the gas and let the engine drag slow the car down. I checked the "play" on the axle, doesn't seem extreme. I checked the shock mount, the transmission mount (mustache bar?), the points where the engine bar mounts to the body, etc...

It sounds very solid, not body related, and I'm out of ideas and plan to crawl under her again today to try to figure it out... I'm concerned to the point where I can't drive the car again until I nail this down (literally (IMG:style_emoticons/default/laugh.gif) ) HELP!!

Thanks in advance,
Chris