i have slippage in my transmission or something
Isn't this a "cop car" with extremely conventional equipment. There is a rear wheel driveshaft and conventonal pumpkin differential, if I can envision this correctly- everything drivetrain wise, old fashioned and well established.
Take it to a transmission shop. It might need the fluid and filter replaced.
Search Mercury Grand Marquis Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Mercury Grand Marquis Experts