Well, I'm not even trying to animate the track skins anymore at the moment. I deleted the "junk" code I was using earlier. I figured I needed to get the tank moving around properly first, then figure out how to animate the track skin and make it match the movement of the tank.

The tank now is properly acting as a physics object though, which is much better than before, and the tracks stay on at the right place. Oh, I also fixed the problem where toggling the engine made the camera unlock from the tank. I needed to include the camera update code in the while(engine_state ==1) loop in the tank's action. But here's the thing bugging me now:

How to get the camera to "chase" the tank, and pan with it, staying directly behind it at all times. In addition to this, the vector of the forces I'm applying need to be linear, and in the same direction the tank is pointing. Right now, the force just pushes the tank in the +x direction, even though I've tried vec_rotate. The camera is also screwed up. It does follow the tank, but when the tank pans, the camera pans behind it, and you lose visual on the tank. I haven't figured out how to make the camera pan while keeping the tank in view.