What make and model truck? Location? Third member, or differential gear carrier assembly repairs can be pricey and mostly not the problem. Verify, verify, verify the concern to make sure it is the problem before diving into unknown areas of repair. A typical ring and pinion gear set costs around 400 or less, plus axle bearings and seals, lubricants and include everything you need to make it as new. Labor ~$360. You can buy a complete used serviceable unit for <$150.