I did the swap you suggested, that confirmed it's a mechanical problem.
If I try to manually turn the tracks by hand, the slow side is a little stiff.
The difference in stiffness is so little it's amazing it could affect the steering of the tank.
My all plastic, all stock, M41 drives 100000000000000x better.
Looks like I'm just gonna have to stop before steering for now.

Thank you ausf for your assistance.