Start cheap? Remove rear pinion input flange, remove and replace rear diffrerential seal, reinstall flange and hope for the best.
Best practice? Disassemble differential, inspect bearings and races on pinion replacing parts as necessary, replace crush sleeve (if so equipped) and rear seal, torque to specs and run down the road.
Doing it right yourself, $100-$150 dollars for parts--add $2-3 hundred dollars for someone else to do it for you.