Cant be that, it's behind the passenger seat within easy reach

Seriously, when the trans down shifts?
Not on up shifts?
My 1st one did that back in the 70's.
Couldnt here or feel on accelleration do to 2.5" pipes going thru the gutted side pipes
Thought it was trans or ujoint.
Found rear joint worn, so replaced it.
Now i'm go'n from beer gut memory as Fbody69 says...it helped, but still had a clunk.
Wanting to go faster, installed a TH350 and 12 bolt rear.
Clunk was gone.
My buddy put the ST300 in his car, and claimed no clunk.
Decided to look at the case/ring & pinion before selling the 10 bolt.
Turned out to the pinion shaft had egg shaped the case, and was moving back and forth.
Any way, that's what i think caused mine.