Dude, you are management material. Not going to school was excellent preparation, but being locked in a closet clinches it. Seriously though, it sounds like you've done well for yourself considering your background. Usually, a roll-away is a terminable offense at Express, but I've seen several RTD's (Express semi drivers) get their jobs back.
The key to your situation remains whether or not others have done the same thing, but received different discipline. It happens all of the time at Express, and since they have basically exported the Express management model over to Freight, I suspect that you wouldn't have much difficulty finding a "favorite" who got treated better than you did. I have little doubt that your age is a factor. FedEx actively tries to eliminate older workers, which is another angle you could approach this from. FedEx often isn't very smart about the law because they have stupid managers who don't understand it.
Check other terminals, and get a lawyer if you can afford one. He/she can subpoena FedEx personnel records that could support your case. Fight them..and win.