I took mine to a good trans shop who did a free visual inspection (for leaks, etc.) and a drive around assessment. Since mine only slipped when it was making a shift (going from 1st to 2nd was the worst) - it had NO slip once in a gear - he concluded the band clutches and other internals were probably okay.
The most likely culprit in his opinion was the modulator (screws onto the right side of the trans). I bought a new one but never got around to installing it as a result of the transplantation of the 5.6 engine and transmission. Hence, I know not if that was my problem or not. There's also supposed to be a way to adjust the modulator for harder or softer shifts, but I didn't get around to figuring that out, either.
He also told me the trans has a limp home mode (stays in 1st gear) so you'll never be stranded. Of course in an early SL that will limit you to 90mph (!), but what the heck - so you're a little late to work.
BTW, I'll soon have my 4.5 engine, trans, and modulator for sale - probably on eBay.