I don't know if my answer is relevant, but I had my 300SD down for about 4 months, procrastinating on rebuilding the front steering. Anyway, when I got it all back together, the trans acted like it was shot, not going into gears, slipping, etc. But it was fine before I parked it. So, I just figured something got stuck in place while sitting. So, each time before I drove it, I made sure it was warmed up well and I drove like an old lady. In about three days, all was back to normal.
Worked for me.
BTW - I don't think it would be the flex disk, as that connect the transmission to the driveshaft - you'd go nowhere in either direction if it failed completely. Usually, they let you know they are failing because of all the vibration they cause when going bad.