If your using a standard sized batter pack, it is unlikely your ESC is sending too much power to the motor. With 5 cells, there's only a little over 6 volts at full charge.
I would look at the drivetrain without a motor. Does it move freely? Compare it with yours if you need a reference. Also, make sure the mesh between spur & pinion has a little play. If it doesn't, there will be excessive drag on the motor. Check it in small increments all around the spur, as most spurs are not true.
One other odd thing..... you didn't by chance replace any screws, did you? Screws mounting motor too long? Screws mounting drivetrain parts somewhere? (This would show up with a bound up drivetrain.)
You could also swap ESCs with yours, to prove the ESC is not the problem.
My suspicion, a drivetrain problem. Maybe a blown bearing, or dirt or something in the front/rear diff.