Hmm...unfortunately the additive is probably what did your tranny in....even if they tout "safe" for all transmissions. It's supposed to make worn friction disc stick and engage to eek out a little more life outta a worn transmission. If it was rebuilt 80k....it actually may have worn those friction disc out faster...causing the friction pad particles to clog up your transmission.
These transmission (well any transmission) are very sensitive to the fluid type and quantity. What did you additive did you use by the way (i used Lucas oil in a failing slipping Honda Civic tranny...helped it out for about 4-5months before it went belly up). Was yours slipping?? I'd flush the transmission out, replace filter and gasket (my e320 95 takes DEX III...although people do have other preferences). Let us know what happens.