IF, and I do mean if, it is the rear axle, yes you can have it rebuilt. Chances are it would probably be easier and cheaper to just swap in another axle from a junk truck. The problem with that is that the used axle may have the same issues or develop them in time. The problem with rebuilding an axle is that they require special tools and knowledge and it's usually not real cheap in parts or labor. I personally do not like the Chrysler 9 1/4" rear axle because problems are very common with them and usually return even after a rebuild. A lot of people deal with the typical "Dodge clunk" and other pinion and backlash issues and get many, many miles out of the axle but I've had a few fail and now I refuse to own a truck with one in it. The issue with swapping a different model axle under there is that you will normally have to swap to 8 lug axles or do something custom.
Now having said all of that, your problem may not necessarily be in the rear axle. It is possible for a ring and pinion to make noise at a particular speed but usually it changes with speed and is there in some way regardless of speed. It's very possible that the noise is related to something else but honestly it's very hard to diagnose these things over the Internet. It could be something as simple as a catalytic converter that has failed and has broken pieces rattling around in the exhaust or it could be a transmission, axle wheel bearing or transfer case bearing going bad or a hundred other things. I wouldn't rule out the axle bearings just because someone "checked" them when putting tires on. Unless the bearing has already started destroying itself, there isn't much that can be determined without pulling the axle shaft. Basically without some piece of info that would help pinpoint the problem better, you are probably going to have to diagnose it further by process of elimination or take it to a knowledgeable mechanic to diagnose.
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