Check your ummmm..... Universal Joint Cups. There are 4 that I can tell. They are all in the centerline of the chassis. Make sure they are screwed down tight on the shaft.
Here is an example:
pick the truck up, turn the left rear tire only(don't hold the right rear tire), and the right rear tire should turn the opposite direction. Same way on the front tires.
If you hold the right rear tire and spin the left rear tire, the front tires(left or right or both) should turn also by just spinning one of the rear tires.
If they don't you have a drivetrain problem. Also check the universal joint cups on the main gear assembly(the clutchbell turns this). There is one on the front of it and one on the back of it. Make sure they are tight and not spinning freely on the shaft.
If that don't solve it, I don't know what else to tell you unless you tear the gearboxes down and see if something is broke or missing.