What is the condition of the bearings?
Not uncommon for a seized bearing to generate heat and melt plastic fairly quickly too
If you ever have a failed bearing on a hub, then you must also replace the plastic hub carrier otherwise slop will form and you will repeatedly go through bearings on the same hub.
I would also check gearing and motor temps... a hot motor can melt your center diff:
PRIMER: What gearing do I need to run for upgraded electronics in my car?