On my former brick, '86 245 auto tranny, it gave a good thunk at some shifts.
I put up with it.
Eventually it developed a leak at the transmission rear tail housing seal + bushing. Good mechanic said to replace both. We did. That cured the leak and also the thunk.

The bushing is metal, when it goes the output shaft doesn't stay in line. There's a rubber seal there too but the bushing is the real seal that keeps the fluid from escaping. You feel for play there, grab the output end of it and push up/down or sideways. I can't tell you how much play there should be but my mechanic sure knew when it was bad.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.