Torque convertor?

If I were you, I'd have it towed to a transmission shop for diagnosis, hopefully by a friend that's willing to put it on a dolly (you can rent one from U-Haul for cheap) so you're not at the tranny shop's mercy for towing charges.

And, if it's possible, I'd take it to at least 2 transmission shops and I wouldn't tell either that another shop has looked at it.

If you do that - Let us know what they find!