Sorry if I get this wrong , haven't the chance to read all this stuff yet...

on "running out of room" I was referring to room on the bracket on the axle needed to oval the holes out enough. The hole in front of the axle would bottom out the trailing arm inside the braket before you could move it much.
The rear hole gets ovaled downward. Not much metal left on the braket not cutout. I was going to weld a washer from a trailing arm onto the braket. The hole is exact size for the bolt.
I have not done this project yet. just alot of measuring.
I just put my 13mm lenghtened driveshaft in this month. If this doesn't stop the vibes ,,,then I'll do the pinion angle change.
The guy that made my driveshaft said there wasn,t enough change to make a difference. I think it does matter.

Now that's funny. <img src="/forums/images/graemlins/lol.gif" alt="" /> <img src="/forums/images/graemlins/ignore.gif" alt="" />

Ok, I assume a 2.5' trailing arm length, which becomes the radius of the arc (the circle) that the axle makes as it is moved down to lift the rear with the gen2 springs. This means a circle of diameter 5' x pi for the circumference of about 15.7', or 187", divided by 360degrees in a circle equals about 1/2" per degree, very roughly. If the axle centerline is 5.5" from the trailing arm bolt hole, that's a 11" circle, or a circumference of 36ish ", divided by 360*, or about .1" per degree. You are out about 2*, so .2" hole movement.

Your math is correct...but I think the measurement points need to be different.

Making a couple of assumptions:
-The output of the T-case is a fixed angle based on how it's mounted.
-The angle of the trailing arms (in a neutral position) is fixed due to the weight of the truck (and Larry)

Basically the axle needs to be rotated within the trailing arms, and the length or angle of the trailing arms doesn't affect the math at this point.

So, really the measurement depends on the distance from the trailing arm "pivot" bolt on the axle, to the pinion flange, and the distance of the slotted "adjustment" bolt to the "pivot" bolt. After that you should be able to calculate it the same way.

A way to visualize would be like a step ladder...The distance at the feet is greater than at the hinge, but the angle at the top is the same. The hinge is the adjustment bolt, the feet are the pinion. A small movement at the hinge results in a big movement at the feet, but they still have the same angle.

You 2 are just showin off now.

I need to rotate the diff downward, based on my angle measurements. I have about 1 1/4" of useful space to cut on before I feel like I'll be compromising the integrity of the trailing arm bracket. I follow both of ya'lls cipherin, and I can't figure out if ya'll think I should or shouldn't. But if nobody tries it, we'll never know if this fixes the issue...

Nate, the trailing arm calculation is just to illustrate how much angle change the spring lift/axle drop causes based on the length of the articulating member. The correction must come from somewhere else, or you have to reverse the lift. There are a couple of obvious choices for the correction method, one easy, one hard. The hard one is to cut off all the axle mounting hardware (panhard rod, trailing arms, spring perches) and reweld them at the new location with the angle corrected. THe easier one is to notch one of the two trailing arm bolt mounting holes and rotate the axle to correct the angle. the drawback of this second method is twofold. One, the spring is slightly canted on the perch, and second, the panhard rod is in a slight bind on it's pivot.