Could the rear end be locked. Back tire on driver side want move

How long was the noise going before the failure? There is a known issue with excessive rear axle wear on these cars that can lead to fluid leaks, and ultimately failure: http://repairpal.com/wear-on-rear-axle-shafts-may-cause-excessive-play-and-gear-oil-leaks-744 There were axle-related recalls for the Crown Victoria and Town Car, but I don't see anything specific to Mercury.