Last year my car was fine, I had a small clunk which I tributed to the crappy 10 bolt(3.73) gears. I've had the car well over 120MPH with that rear without issues. I have the MWC body mount torque arm on the car for the entire time.
I built a ford 8.8 rear for the car, I used the stock axles and the hiltsy mount, i bumped it up to 4.11 gears. I also installed PHB and LCA. The car had a decent vibration at 80MPH and would get worse as I sped up. Assumed I f-ed up the drive shaft when i put the u joint in. This brings us to this year. I had a pretty bad oil leak and I fixed the oil pan gasket, put a BMR k member in the car and I replaced the stock shaft with PST shaft. Now the vibration starts at 50-60MPH and by 90 I feel like death is near. Its shake like crazy. I'm thinking the PST shaft is just more solid so the vibration is sooner and more intense.

I've owned maybe 15 f bodys over the last 23 years I have never not once had one do this. Not one, I've used every torque arm on the market, had 12 bolts, 9inch rears stock shafts after market etc. I've never had one do this.

The 8.8 was built from a f150, I used stock axle tubes with a custom sleeve to take up the gap, stock axles. The mount is 90 degree from the pinion just like stock and the axles are clocked correctly. I used a truen bar to make sure it was straight. The only issue is the PS was .250 short and I had to ad a spacer to fix the brakes.
I used Ebay posi https://www.ebay.com/itm/266824253397 and used QTP gear set https://www.ebay.com/itm/224005906953 solid crush sleeve. I've built a decent amount of 10 bolts and a few 12's.

I'm not sure where to start without tossing tons of money at this.
rear nitto tires are bad?
Axles are bent and have gotten worse?
Carrier is out of balance? Not sure if this is even possible
The gears are bad?
Could a bad pinion bearing cause this?
Buy another 10 bolt and install it?

 

Bent axle?

 

Have you checked your driveshaft angles?

 

I'm currently negative 2 degrees. I measured on the base of the trans and then the torque arm mount.

 

I pulled all four wheels and had them balanced, then swapped back to the stock driveshaft. The vibration is still there, but it’s about 60% improved and now mostly shows up on decel/low load at higher speeds. Looks like the stock shaft definitely helps smooth things out.

Next, I tore down the rear end and started a full rebuild. The gears weren’t set up correctly — pinion depth was off, and there was a strange wear pattern on the coast side. The 4.10s are out and I went with a set of 3.55s. Even if this doesn’t completely cure the issue, it should at least move the problem past 100 MPH.

I checked the axles and they seemed fine when rolled, but I went ahead and ordered new Yukon axles just to eliminate that variable.

I’ll post another update after the gears are broken in and I get in the first 100 MPH test run.

 

Everything is smooth now after the gears and axles.