Well, that's one of the single motor units with a couple of geared axles with a U-Joint, so it's quite possible that's the issue.
I suspect if the U-Joing separates, that truck would be pretty hard to push with that worm gear engaged. It doesn't seem that this would be a very difficult repair, the drive shaft is in stock for $12. That would be $6 during the 1/2 price sale coming up, buy a couple of them, the engine has two.
If they keep breaking, that engine looks like a candidate to kitbash with a metal U-joint instead of that plastic one.
