I currently have a 9 bolt in my car that has a shot posi. The gears are really loud (they make a constant whirring sound that gets louder as speed increases). The gears also don’t sound very happy when I engine brake (very loud gear sounds, kind of a loud wirrr that trails off as the car slows down). Some the engine breaking sound could transmission related (I wouldn’t know ), but a lot of it seems to be coming from the rear end. I had the rear seals and bearings replaced about a year ago and it was hard to find tge seals. The machine shop had to spike the axles (if that’s what it’s called when they hit the axle with a chisel right in front of the bearing to keep it in place) because the axles were worn.

Recently the rear developed a low-pitched whirring sound (almost like a vibration but without any noticeable vibrating?) coming from somewhere in the driveline when cruising at around 50+ mph. The sound osculates louder and quieter despite no obvious changes in throttle/speed. The sound goes away if I put it in neutral. Also from time to time when I hit the gas I can hear a click of something engaging when I go from coasting with no gas to lightly accelerating – when the engine comes up to speed (1,100ish rpm to whatever it should be at in that gear) and the transmission catches there’s a pretty clear click. It could even be the transmission mount, which is shot too, but I doubt it. I have the feeling that all these sounds are related and that it probably means that the gears are on their way out and have a pretty good amount of play in them.

The rear most likely has 104k miles on it. I doubt that it has been rebuilt by any of the previous owners.

I went to a local speed shop type place and they found everything I need for a complete rebuild. It will cost roughly $900 in parts to get it up to factory spec (new seals, gears, posi unit, the basics). I don’t remember if a carrier was involved in that cost, but I think so.

That $900 is about half the price of getting a 12 bolt/9”. Given how rare parts for 9 bolt are right now, I can’t imagine how hard it will be to find these things in the future when it’s another 10 years older. Planned engine mods will only shorten that life. I know that the 9 bolt is supposed to be relatively strong, but it won’t last forever and I will need to rebuild it again eventually. When that time comes, I’m worried that I wont be able to find the parts again.

This leaves me with two choices: rebuild it anyway knowing that I probably won’t be able to do it again or swap rears. I’m shying away from a 4th gen rear because of the 4” width difference. I’m also shying away from the 10 bolt because it’s not that strong to begin with. A 12 bolt or 9 inch seems a viable option because they’re really common, plenty strong (sure you can break them racing, but I don’t ever plan to run slicks even after the engine mods), and I won’t have to worry about having to work on it for a while.

I did a search and I didn’t come up with much useful information in this regard. I project the cost of a 12 bolt/9” swap to be around $2,000 complete with everything needed (including posi) and all new parts. Is this realistic? Also, what’s involved in swapping to one of the two rears? Which is easier to swap, etc. How either rear mount to the LCAs? What about suspension mounts, torque arm, pan hard bar, etc? Will I have to get custom axle tubes and axles or will I get all of this in some kind of kit when I order the new rear?

Any advice is more than appreciated

Sorry about the long post