Take the pinion off the motor and check all wheels run freely by hand. Then remove and check all universal CV's are in good condition. Also check there and no notches in the out drives.
When I've had this same problem it was a bound or broken universal.
. . . - - - Now enjoying Proline tyres - - - . . .
Scale race models: RCHobbyzone, Mugen, Reds, Xpert, Contact, Neo+.
Toy cars: Xray T4'15, Hudy, Nosram/Muchmore, Protoform.
No longer defined as a Off or On-road racer, but simply a RC enthusiast.