BB,

I've BTDT with the same problem. All of the above advice is correct, you just won't know until they start teardown. If the piston is the culprit, say thank you and hug your checkbook. If the band is broken, they will remove the piston with the car on the lift and the tranny installed, then say that's not the problem, we have to pull the tranny.

Once they pull the tranny, they'll have to pull the guts completely out because the band you probably lost is the on the rear set of planetaries. Once they get this far, you have to decide whether to rebuild or install a commercial rebuild. You'll have already paid for the labor to teardown the original. I chose to have a mechanic I trusted rebuild mine, partly because I knew what I had and partly because I figured they wouldn't have too many 4matic trannies on the shelf.

I had the whole thing done, including the replacement band, for about $1900 with all new parts. YMMV.

Good luck!