OK...... SO.....
I took my truck all apart last night to check everything I could. I swapped out the drive shaft and pinion gear in the process because I had purchased spares a while back. I tested it in the road this mourning and it seems to be working fine now. I think it may have been something to do with the pinion gear after all.
Thanks for all the help again... Now I can get back to work on building my track.