1999 Plymouth Voyager HELP!

Transmission problem
1999 Plymouth Voyager 6 cyl Front Wheel Drive Automatic 220K miles

I had to leave my van parked at a gas station off the highway last night because.

I pulled off the interstate for gas. At the stop sign I heard (felt) a modest thud but the vehicle continued with no issue. As I pulled to the gas pump it sounded like I had just rolled onto gravel (big gravel). I thought it was my radio. I shut down, gased up and started to leave then knew I had trouble. The engine sounds fine. It is when I move forward that it sounds like a giant eating marbles. I was afraid to destroy the van so I parked it and got a friend to come get me. Now I have to make some decisions, but I need to knwo what I'm dealing with. Can you help me? Do you think it is transmission or drive train related? I'm stuck out of town and have to get back home quick. Help!