Left rear of mini crawler does not pull positively

[mwe-maxxowner]

I recently bought a used losi mini crawler, and so far I love it. I seem to have a problem with it, though. Aren't both the front and rear supposed to be locked? My front is fully locked, both wheels pull all the time, and you cannot spin an individual side independently without turning the whole drivetrain. My left rear, however, I can grab with a hand, and spin it independently of the rest of the tires/axles/etc. It also does not spin when the truck is under certain conditions while driving. The right side always pulls, but sometimes the right will be spinning while the left is the only one on the groung...not turning. What could likely be the culprit here.