Optima - Servo Mount

[NegativeGhostRider]

The wheel drive hexes on the vintage cars have a taper fit to the axle shafts. If they are slipping, it might be due to a loose wheel nut. The drive hexes on the re-release cars are pinned to the axle shafts so they can't slip.
Assuming that the dogbones and other drive train parts are in good order, you might check the grub screws that secure the drive cups to the differentials. If one has backed-off, it could explain why only three wheels are spinning.

I am guessing it is the set screws and it is probably just a bit loose. The wheel turns sometimes, but slower than others. Also, when it doesn't turn, the dogbone doesn't turn. I'm going to open up the gearbox and check it out.

[XLR8]

Okay but you can access the drive cup grub screws without pulling anything apart.

The fronts are readily accessible but the rears will require that you to slip your hex driver between the transmission case and the aluminum plate.
It's tight quarters but you can gain access to the screws without taking the car apart.