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It does not seem likely that you could have a bent axle without damage occurring internally from extended use, however it is possible.
First, consider any other possibilities. It is very common for this type of wear to occur on the front of the vehicle, so is it at all possible that the tires were worn like this while on the front and then rotated to the rear. Perhaps this could have happened before the tie rod was replaced?
Has your mechanic measured the rear axle? Or did he just suggest it as a likely possibility. A wheel alignment would be the quickest way to find out for sure. A modern alignment machine will give a printout showing exactly what is out of spec or bent.
When you take the vehicle to have the alignment done, just tell them the symptoms that you are having so that the mechanic doing the work knows what to look for. They may want to charge extra in some cases to set up a 4 wheel alignment, but most of the modern machines will show the rear readings no matter what.
If the axle IS bent, it doesn't necessarily need to be replaced. Many of them can be straightened if they aren't too bad, and sometimes it can be done with the axle still in the vehicle. If you have a set of torches, and some basic repair knowledge, I might be able to walk you through it. It's more science then mechanics, but slightly bent axles can be straightened using the expansion and contraction of heat to "warp" it back straight. This is perhaps not a common repair, so a mechanic may give you a funny look, but anyone who races cars and crashes once in a while will know what I'm saying.
Hope this gives you the information that you need.
when you say that you had a 4 wheel alignment and it doesn't hold, are you saying that they are making an adjustment of some sort? OR are they simply moving the alignment head on the wheel to make it show that it was changed. There is no way to adjust this without bending it or replacing it.
Before it can be straightened, you will need an alignment printout that shows which wheel is not pointed straight. If the axle is bent out by the wheel, it may just show one wheel toed in, but more than likely, the housing will be bent closer to the center. And since the center is in between the mountings, it would cause both rear wheels to be toed in and not just one.
Once you have that printout, or even if you want to skip that step, then you need to get the vehicle off the ground and take your own measurement with a tape measure. Find a good spot across the front of the tire tread to measure in-between on each tire. mark this spot with something because you need to use this exact spot on both tires to take your measurement. You're going to measure between your marks on the front of the tire at exactly the height of the center of the axle. Then, (vehicle in neutral) rotate both tires 180 degrees and repeat the measurement on the rear of the axle using the exact same lines.
If your tire wear is on the outside edge, the measurement on the front of the axle will be less than the measurement on the rear. If that is the case, you will want to heat up a spot on the rear of the axle tubing about the size of a small apple until it starts to turn orange. this will cause the axle to toe in more because of the metal expanding, but then as it is allowed to cool naturally, it will contract more than it is able to expand, and therefore bring the toe outward slightly. Allow the housing to cool 100% before you measure it again. If it didn't do anything, heat up a slightly bigger spot. If it helped a little bit, maybe heat up the same size spot again.
Judging from the pics I have of the axle, the RH axle tube is more likely to have bent. the left one looks shorter and stronger. so I would do it on the right axle tube about half way between the center section and the suspension mount.
there's going to be a lot of smoke from the axle lube burning as you heat it. there shouldn't be much lube in the axle tube, but it might be a good idea to drain it out just in case. you don't want to burn any more of it than necessary.
I hope this all makes sense and gives you what you need to be able to fix this.