Ok, I've fixed this so that the force now pushes in the tank's facing, rather than just the world's x axis. Major improvement!

Only thing now is I get a little "slip n' slide", or "yaw/pan" as the force pushes the tank around now (probably due to the friction/collisions with ground as it moves). As you know, tanks can't move sideways, not even slightly, lol. So I wonder how to fix that? I've been giving it some deep thought, but haven't even had an idea worth trying yet.