Vibration right off idle
3° down at the rear end pinion angle as compared to the tailshaft angle sounds correct if memory serves me.
3° down under load comes up to 0° when you put the power too it and get on it.
Bump stop above the rear yoke is a safety stop to keep the rear angle from twisting up to high under load and taking the rear u-joint out.
Been a long time ago in the 80s when we were setting up new length drive shafts for extending frames and sleepers for semi-tractors, those were set at 1.5° down.
In my opinion, double check some more info and or call a driveshaft shop.
Good Luck
You don't want those driveshaft angles fighting each other. Kind of like some old PTO driven farm machinery when you turn too tight, the u-joints start rattling to beat heck, and is extremely hard on the u-joints.