Is it coming to a stop in a straight line, or cornering?
If you hold the car off the ground, give it full throttle and let off. Does it seem to coast or have brake effect?
Years ago I had a on LRP Sphere that had an issue, it behaved like it had mega drag brake, undriveable. Had to send it in for repairs. If you can, swap ESC and see if it does this still. And make sure you don't have any brake trim on your radio.
As long as your drivetrain is free, I can't see it being the diff. Unless again, you are talking about slowing in corners only. Then I'd say try heavier front fluid.