I really would not drive it again until you do the fluid. It will only take you an hour. Go get 4 quarts of synchromesh ($8/bottle) and get your car up on ramps. Pull the drain plug and let it totally drain out. Re-install plug, go inside car and pull shifter, slowly fill to 3.2 quarts, re-install shifter and take it for a spin.
Honestly one of the first things I say anyone buying a 3650 equipped car should do is get the old fluid the hell out of there and re-fill to the updated spec because VERY often they still are on original fluid that was overfilled from the factory. Old broken down fluid that is overfilled in a trans that was known to have 1st run issues is a very bad combo.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.