Ford Repair: Ford Crown Vic Diff problem, ford crown victoria police interceptor, ford crown vic

Question
I hope you can help.
I have a 2002 Ford Crown Victoria Police Interceptor and have just started to have this problem.
When I put the car into Drive it MIGHT move two or three feet there is a "thud" then then a loud grinding from the rear (I think the diff).
It works a bit better in reverse but it happens again.
Any ideas what might be causing this, and the CHEAPEST way to get it fixed.
Thanks

Answer
hry there, the cheapest way out of this is to get a used diff from a junkjard, that grinding noise is definitly going to be desrtoyed spider gears. and all the steel filings from the broken gears are going to be all over, so you will also have to do bearing, i recomend you just install a used rear end for the cheapest way. good luck
Chris P