Trying to solve this for a year now

[Paisano]

1992 Ford Ranger XLT with 2.9 V6

The problem is pulsating and 'washboard dirt road' vibration when I press the brake pedal. I have tried everything.
These are the areas I have checked more than once:

front brake overhaul done 2 years ago. I do all my own work and troubleshooting at home.

inspected rear drum brakes
inspected front disc brakes including bearings and dial gauge on rotors
inspected tires with rotation included
inspected steering linkage and steering shaft
inspected front suspension
pedals and steering wheel test for motor mounts......normal result
pedal tests for brake booster and master cylinder......normal result
bled brake lines
removed rear anti-lock brake valve and bypassed with new brake line
disc brake caliper replaced after it hit concrete floor hard.

Notable observations:

Earlier this year emissions testing station runs up engine with rear wheel on that roller treadmill thing.....and front wheels stationary
No 'washboard road' vibration

I run up engine with rear wheels on jackstands.........no pedal pulsating......no pedal vibration

This evening 5/26.......drove truck in drive and reverse in large empty parking lot
pedal pulsating and washboard road vibration felt in either direction

front driver side wheel checked again May 24
spindle axle stub has maybe 1/16 inch gap behind snap ring, where thrust washer and/or spacer should be.
I already have Spicer washer kit

I'm thinking front brake overhaul in very near future. What do you all think?