Ok... I have read way to much on pinion angle and setting it with all the weight (fully assembled) of the car, tacking and removing rear diff to finish welding, etc, ect, ect...... This should be simple. With the perches level, as mounted in the car (under the tube) what angle is the pinion at? I've heard everywhere from +5 (pointed up) to -5 (pointed down). I know the perches are not level when installed, but it's hard to believe that a +5 ends up being a neg pinion angle once installed. Does someone have a stock rear they could measure? Ford built WAY too many rear diffs to not standardized this and jig the angles for welding. I could answer my own question but I sold the 8" out of mine and this was the one thing I forgot to measure and I'm not sure truck angles ( from this Explorer 8.8) would be the same.
If you have changed things on the car, I wouldn't trust the stock angle. New leaf springs, different shackles, changing the ride height, motor swap, or different motor mounts can all play into this. The stock angle may get you close, but I would rather work off of your particular combination. If any of those items have yet to be changed, and the car doesn't need to be driven, maybe tack the mounts in place, and do a final revision later? You want the pinion angle to be parallel to the transmission output angle. For example, if the trans is pointing down 2 degrees, you want the pinion pointing up 2 degrees. I don't think the curved leaf spring is level with the ground where the axle mounts to it. The front eye of the spring tends to be mounted lower on most cars than the rear eye. This would explain the discrepancy between the mount angle and pinion angle. Interested in the Explorer axle swap. Did you narrow it with two short side axles? ... or some other combo?
I'm pretty sure that when ford made all these housings, they didn't know if it was going into a i6 or a 302 car, 2dr or 4dr. The center sections were added later with the axels. I have never seen shims in an original car, so they must have had an angle that was acceptable for all. And yes, doing 2 short side axels.
They also built these cars somewhat by hand and the body might be off by a half inch one side to the other.... "good enough". If you want "good enough", by all means go for it. No offense taken, but what I outlined is how to do it "right". Chasing a shake is no fun. I did it for months with one vehicle .... two different issues, motor mounts, and new leaf springs. Ironic to the conversation, that was an Explorer. It was maddening at 62 mph. Fine below it, got fine again at 70.
I set mine at 3* down compared to the trans output shaft to account for the way the pinion gear rides up the ring gear during acceleration.
Download the Tremec app for smart phone, follow the instructions on checking the three angles, pinion,driveshaft and output, it will tell you when you have it right, If you set it up for drag racing you will have a vibration from hell cruising at highway speed Captain Comet is correct if the pinion and output angles are not the same or very close it will vibrate, the third angle in this equation is the driveshaft and that the output is or should be slightly higher than the pinion and these two planes should never intersect. with the app you will check output then driveshaft and finally the pinion, it will then show you angle 1, angle 2 and drive line operating angle. all three must be green, if any are red you need to change the pinion angle. I used it and added 2.5 degree shims and now is smooth sailing on highway, If you do get a lot of axle wrap under hard acceleration you probably wont notice any vibration since it is less noticeable under load than not
It also has speed, RPM and tire size calculator, there are angle finder apps, analog and digital angle tools but then you have to do the math
