There is no pinion angle adjustment on the OEM torque arm...

 

The drive line angle could still be the issue. The angle in the front of the drive shaft needs to be equal to the angle at the pinion within 1/2 deg. Longer driveshaft will effect the angle as would a shorter transmission. Also the weights do more then most techs think when dealing with picky people. I have even hung weights as little as one pound on exhaust systems to eliminate light throttle vibration. Course that was driven by a GM bulletin not me figuring it out.

If you know someone that knows how to set driveline angle you can make small adjustment by shimming spacers under the cross members to move it down and under the mount to move it up.

Not the best video

 

^ Interesting, and along the lines of what I might assume is the issue, but I am at wits end...

That video was kind of pointless, they found some differences 1 deg between front and rear, but did not say the meaning or what is in tolerance... In fact they said NOTHING, and almost implied 1 deg is acceptable. *rolls eyes*

Do you think the subframe connectors I had welded on a two point lift, instead of when the suspension was at rest (four point lift) could be a factor? Happen to have that GM bulletin handy?

 

The one pound weight to the exhaust bulletin was not for an F body. I was just trying to point out that the weights do effect light load vibration. It was for an 89 Astro Van. It satisfied about 90% of the vibration complaints. The other 10% should not have bought a van in the first place.

The point of the video was to show you how to measure. The ½ deg spec I quoted was GM spec in the Late 80’s. U joint is still a U joint so I doubt that has changed. I was trained by GM on vibration diagnoses which meant I got stuck dealing people that where very picky. There was nothing worse than former Cadillac owner that was forced to down grade to a Caprice. I put a lot of washers under cross members back in the day.
FYI: one deg on a lifted truck with big mud tires on it is probably acceptable. The tires themselves have a fair amount of vibration. It is not uncommon to see 2 or 3 degrees in lifted truck. Once you pass 2 degrees the stock U-joint itself will have a limited life.

 

In this case, no. The longer and shorter transmissions have the crossmember mounting pad the same distance back from the bellhousing, so transmission angle would remain unchanged. His driveshaft angle may be slightly different due to it now being a little longer, but that has no effect on pinion angles.
With that said, there still could be something throwing his pinion angles off whack; worn control arm or torque arm bushings, for instance. Does this car still have the stock rear suspension components?

 

how can that be if you use both the angle of the pinion shaft and the angle of the driveshaft to figure pinion angle setting.

 

Pinion angle is the relationship between the angle of the transmission output shaft and the rearend pinion shaft (you check that at the yokes), with or without the driveshaft in place. Driveshaft angle doesn't change pinion angle. You could have a driveshaft at a 5 degree pitch or a 45 degree pitch, and the pinion angles would still be the same. Pinion angles being out of spec is what causes bind and vibrations, not the driveshaft angle.

 

to get true pinion angle you subtract the pinion angle from the driveshaft shaft angle. mine has to be just right to not vibrate. the mount keeps bending/shifting and it vibrates over 70.

 

Well yeah, that's right. It's better to have the optimum driveshaft to pinion angle, but not all cars can get there especially if they have a real short shaft. The angles between trans output shaft and pinion shaft want to be nearly parallel to each other. A rule of thumb with leaf spring cars is for the rearend pinion to have a 1 degree angle down to compensate for torque.
Did you have to play with trans mount heights to get yours in spec? Another thought, I wonder if he changed his trans mount to one that has a different height from stock?

 

my motor mounts were all stretched so it sat 1/2 inch or so lower than it should. replaced them and a lot of the vibration when away. still does it when things get outa whack