I have a 01 Ford F 150 with 320K miles. I love this truck and don't plan on ever selling it. I thought the bearings were going out in the back right side so I took it to mechanic. Two different mechanics say the rear end needs to be rebuilt or replaced. One says if I'm going to keep it I should rebuild it. Second says I should replace it with a used one (rebuilt never works). They can both do either and are about the same price. Rebuilt has a warranty but used one will have warranty for only 45 days. One can tell me the used one has 130k miles on it the other doesn't know because its already pulled. What is recommended rebuilt or replaced??
My suggestion would be to buy a differential bearing and seal kit for the rear end yourself and have a knowledgeable mechanic install it for you. This, as the name implies, consists of bearings and seals. A master kit will have the shims in it as well, but when you are just doing bearings in the diff, you can usually put the old shims back in to get everything back to right. You'll want bearings such as Timken or Standard Gear which have great reputations. The reason this is my suggestion is, if you buy these parts, you know exactly what has gone into the diff. If you rely on the mechanic to get you the parts, they could get the cheapest thing they can get away with. These parts will last long enough for their warranty period, but you wouldn't want to trust them much longer.
@JasonJanowitz is right in that getting a used one is a crap shoot. It could be 6-months or two days away from needing a rebuild ... you just don't know. Getting your rear end rebuilt gets you back to exactly where your truck needs to be. There's no guess work. Just because you have 320k miles on it doesn't mean any of the hard parts are shot. As long as it hasn't gone dry and it hasn't been over worked, they are probably fine. If they aren't, getting a new set to put in really isn't all that expensive and shouldn't cost you much more in labor than a rebuild alone would cost.
You really need a knowledgeable mechanic to do this. Don't get a mechanic "friend" to do it for you as this is probably one of the easiest things to screw up on a vehicle if you don't know what you're doing. It's not something a mechanic can just wing and get it right. Most mechanics would get instruction from another knowledgeable mechanic while they are watching to ensure things are right. There is preload in the front bearing which measures in the lb-in realm (meaning, you tighten the front bearing down, then while it's free, you check the preload to the lb-in range ... some mechanics can turn it free hand and tell when the preload is correct.) The steps involved are not something you'll just read in a book or watch in a video and figure out.
you simply don't have enough information here. If it's really a problem with one side, you probably just need a bearing and an axle. With that many miles, the pinion gears could be worn to the point that you get enough slop back and forth that the axle doesn't ride right on the bearing. If that's NOT the case (and it's easy to test just by grabbing the wheel and pulling in and out on it while the truck is on a lift) then find out in DETAIL why they want to replace the whole rear end. Your symptoms point to a bearing.