installed SS springs and pinion angle is at 0

If the trans is 0 the pinion should be -3 down. (leaf, 4 link and IRS are all different values) As the rear end wraps up on acceleration, the pinion rises, and 0 delta between output shaft and pinion angle minimizes HP loss and parasitic loss. Shims are cheap at 4X4 shops. One way to figure it out is lay a level on the valve cover and jack up rear diff until its bubble up, jackstand rear axle then measure the diff yoke straight up and down with an angle finder. figure how far your off and buy the appropriate shim. You dont want 0 at cruise, supposed to be bad for your U-joint as it doesnt spread the load over enough roller bearings. Any believers?