So my rear axle was developing a bad vibration over 100mph only when coasting, completely goes away when accelerating at all and not present at all below 100mph.
I rebuilt my rear end, and found a heavily pitted carrier bearing race. I also put in 4.10 gears while everything was apart. Testing it out, there's still a vibration over 100. It's much less bad, and is tolerable now but still occurs exactly at 100+. Given it still happens at the same speed with different gears, I think that rules out driveshaft/trans/pinion? That leaves the carrier, axle bearings, and wheels. I don't think it's the wheels since it didn't change with a rotation.

Any ideas what would cause vibration coasting over 100?

 

Dump the carrier bearing configuration with the two piece driveshaft\. Go with a one piece and get a drive shaft loop. The stock driveshaft cannot be serviced. You have to buy a whole new one. May as well make it a lighter aluminum one.

 

should of mentioned, I have been running the aluminum shaft for about 2 years.

 

Try clocking/indexing your drive shaft. Remove the 4 bolts on the transmission side, rotate it 90 degrees and retighten. See in what position the vibration is the most tolerable or goes away completely. Most of the time when it comes to drivetrain vibrations, it's harmonics that cause the vibrations. If you can get the harmonics to cancel each other out, you reduce/eliminate vibrations.

 

Pinion angle?

 

That's a thought. If it gets worse over time, that could be the suspension relaxing. If it's not lowered and the problem didn't just start but rather, was gradual over time, it could be the pinion angle.