The car has sat on jackstands for a few weeks while I work on rebuilding the front suspension, and I came one from work one day and there was a puddle of transmission fluid under it with fluid dribbling out the back of the transmission (when it was not leaking at all before), from a source that is hidden without some disassembly. Any ideas what it might be, rear seal maybe?

Could be the vent if it's been jacked up with the front end in the air for a while. If the fluid drains out of the torque converter and back into the transmission body, it's enough to overfill the sump and come out the vent.

Get the car back on flat ground and clean up the mess, top off the fluid, etc before getting excited about replacing seals.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)