86 300E 90k miles has been leaking ATF. Up on a lift at a tranny shop, they thought leaking came from the rear pump, modulator seal, and maybe the governor. After filling (a lot, maybe 2 qts), saw some leak spots on the shop floor. They figured ~ $350 + parts to seal it up. Any thoughts?

I noted a few tranny flares in the days before checking the fluid level, but shifting otherwise has been good. I hope I'm not fast approaching a rebuild job...

Also, are any additives worth trying at all?

I had a leak on the rear pump on a 91 300e. It was self induced following my work on it. If it is the rear pump leaking, then no additives, silicone, gunk, or sealants. It is sealed with specific metric Orings available cheap from the dealer. Remove the transmount and exhaust hanger and you can get the four allen head screws out to pull the pump and install the new seals. Good luck.