I used the Unity Wheel Colliders, so I think the front two turn and the rear two drive. But the bulldozer really should be using belts to drive. It was kinda fun to work on, so I might see if I could get belt driving, then for sure as long as you didn't fall off you'd be able to get back onto the ledge.
If you only use the upgrades to boost your speed, you can get it to go pretty fast. One thing I didn't explain enough was that you need to use the brake, not reverse to stop. Otherwise you'll fall into the dumpster over and over again.
Thanks for the feedback and playing the game!
Draekdude