Actually.... on a TH350, slamming it in park can strip the teeth right off of the governor, if you are lucky. That practice is really not recommended on any trans, unless you are already planning on tearing into it, and possibly even replacing the case. Since the normal operation of a governor depends on the thing turning, therefore forcing the weights out, stopping the rotation suddenly actually does nothing to improve the operation. Doing that with a Torqueflight can also cause breakage of the reverse band if you stop in reverse instead of going all of the way to park.
Besides the possible problems that can be caused to the trans, consider the stress caused on the rest of the drive train. If you feel that the governor may be stuck, it is alot cheaper to just replace it than a rear end or even a drive shaft!