F100 brake problem

I have a 82 F100 pickup with a brake problem that I cannot figure out.When the brakes are applied there is a pulsation in the petal and a slight shaking in the whole truck. There is no pulsation in the steering wheel which leads me to believe it's in the rear. I had the drums turned but it made no difference. I did check the front rotors with a dial indicator for runout just to be sure and they were good. I can jack up the rear and put it in gear to get the rear spinning and apply the brakes and there is no vibration. It does seem to show up worse if I am going down a hill when braking and I can feel more of a bump bump bump in the petal. Everything points to rear drums out of round but with having them turned and no difference I don't want to just buy new ones untill I am sure that's the problem.I also checked the axles and they are tight and not bent. What seemed to be a simple problem has me scratching my head. Any ideas???

Vibration problems in 25 year old trucks, tough.

Check all bushings in the front end.

Check booster system.

Check hydraulics for leakage.

check the steering links for looseness.

Pulse in the pedal could be the rotors or the drums being out of round. If the lug nuts on the wheel are torqued down too much with an air impact tool it can distort the rotors. Conversely, pulling the wheel it won't be shown in a dial gauge runout test. I always tighten the lug nuts in a criss-cross manner with a torque wrench set to 100lb/ft. Upon jacking up the rear and not having any of the same symptoms would lead me to believe the problem is in the front rotors.