2001 Dodge Ram Noisy differential

Express Tires counter guy told me the noise was coming from the differential. I took his word for it. He quoted $800 to $1300 for fix. I asked him if they were doing the work, answer was they were going to send it out. I went to parts store and priced parts--probably could get out of store for $70 on parts (pinion bearings, cup, seals, and keeping the original ring if still usable). I don't know much about replacing gears in differential, but understand gears need to be assembled within tolerances. I don't understand the huge cost? And don't want to be taken to the cleaners. Any advice on average cost in San Diego, or referral is appreciated.

Thanks, this sounds like good advice. I appreciate the info very much. Any further info on what needs replacing and what doesn't need replacing to save costs while making the vehicle good to go is what I am attempting.