About how much would that cost to get the rear end replaced in a 2002 ford explo

Hello, It all depends on which parts must be replaced. If the ring and pinion gear are the only issue, you could still be looking at several hundred dollars. The price tag goes up from there if other things are damaged, spider gears, carrier bearings, etc.. That being said, you really should bring it to an ASE certified technician for a pre-purchase inspection and estimate for the entire vehicle.