Transmission help.

O.K , This isn't a vintage problem but it is Mopar . lol. Need a Transmission tech that can help. I bought a 2003 Dodge van 3500. It has a 5.9 or 360. I hate using liters. This van has been sitting a while. Drove it out of the yard and went about 3 miles , As I was pulling in to get gas it wouldn't move. Put it in park then back to drive And eventually inched it up to the gas pumps. Had to have it flat bedded home from there. Even in Neutral the rear wheels were locked up. I thought it may have been the ring and pinion but pulled the drive shaft and its fine. This van was a fleet truck, has 221,000 miles but always been serviced. Looks like they put a new tranny in around 148,000. I have all the records. It was takin off the road in Aug,2016 . hopping its not the tranny but a sensor or something of that nature. Any help will be appreciated. Thanks Brian