My 1995 rear wheel drive, 4.9l, E4OD (approx 180k miles) has been working great. Changed the trans filter and fluid about 5,000 miles ago.

I Got home and was ready to back down the driveway, shifted into reverse and the truck wouldn't move. It was a slight incline and it wouldn't move in neutral, shifted to drive and with lots of throttle, it would barely go forward. There weren't any strange noises, just the transmission seemed to be locked up. I disconnected the shift cable at the trans and moved the shift linkage but still wouldn't move. I ended up disconnecting the driveline and coasting back down the driveway. Very glad it happened where it did.

I'm hoping for some suggestions as to what could cause this and if there's anything I should do before dropping the pan or pulling the trans.