RE: RC8T Diff Problem HELP
The shims were definitely what prevented the diffs from spinning freely. Don't overshim as it creates a high load for the motor.
As for the spinning, I have a ST-1 which uses the same diffs and don't have that problem. Sounds like you may have under-shimmed it now. If you're running pretty thick fluid and the gears aren't well meshed due to too much space, then the fluid will be viscous enough to move the other wheel the same direction. Once in a while, the gears gets close enough to each other to engage and actually spin the correct (opposite) direction.
When you shim the diffs, don't just remove all or use all the shims. Play around with what works best. There's 6 gears to shim, so it can take a while. Usually, I just run a shim behind each small gear. But in some of my diffs, I've had to use a shim behind one of the large gears.