If they'll check it out for free, that's great. The problem is that I don't trust them to actually tell you what's wrong with the tranny, and they may decide they need to tear it down to find out. Once that happens, they are in a much better position to negotiate the cost of reassembly, whether or not they fix it.

I'd go to an independant and pay before I'd ever go to AAMCO for free.

I don't mean to scare you about this, but I'm completely serious. As far as I'm concerned, AAMCO does NOT provide free diagnosis. They may say they do, but any check-out or diag that cannot be counted on to be anywhere near correct is worse than paying double for the correct diag.