caravan transmission problem, advice needed

I recently was given a 96 caravan 3.0 3 speed auto with a bad transmission (31TH / A670 Torqueflite). My initial thought was to get a transmission from a local wrecker, but I am unable to find any at a reasonable cost. As I am making this a father-son project I thought it would be a prime opportunity to split the transmission and see what can be fixed. The van was running and driving fine - until it was stuck in the snow, and was rocked (probably quite hard) between reverse and drive to get it un-stuck. After this, the van had no reverse. I drove it today for the first time and it is delayed in shifting into forward gears, but will drive once it engages, and it definitely has no reverse. The fluid level is full My thoughts were a broken band, smoked clutch pack or a bad overrunning clutch. After some research though I have read about bad (plastic) governor pistons in these transmissions which break, affecting fluid line pressure. It appears that the governor piston is the same as the aluminum one used in 904's. I think that I may drop the valve body first and check things out. Does anyone have any ideas or advice?

Thanks