From prior posts I gathered if the trans is 3 &3/4 deg down then you want the pinion 3&3/4 up then lower it ~2 deg from there as the pinion will climb on accel so that'd be 1&3/4 up on the pinion. Maybe the trans being 3&3/4 down is too much to start with & it needs to be raised some for a starting point . I'd wait for the Doc or someone that knows what they're doing to chime in. I had a 1/2 ton 77 dodge pickup used for hauling that would vibrate on decell ONLY when extremely loaded down with debris so yes it was the pinion angle that was changed from the spring load. 8&1/4 iirc. EDIT I reread your post & it might actually be the gearset