[gumby79]

Adjust the pre load. 1+
If there ball bearings then up grade to tapered rollers they handle axial thrust better as well(the direction of bump thrust from pic above ). As the quote said adjusting the preload on balls is only temporary (as the hard crome plating has already warn through ) the balls are physically smaller/not round( probably) replacement is recommended. Call King Bearing in Fresno, Ca hart of ag land they have it on the shelf w/o dealer markups. But by all means inspect repack with" ball Bearing grease". They are difrent never use ball in roller!!! Ball is more of a fibrous chain that the balls massage. Where as the roller wraps them up like sticking a kitchen mixer next to somebody with long hair really quickly destroys the grease it becomes runny and leaks out as oil .
The harmonic that is responsible for the Caster wheel flutter we called tred squirm. Stiffer side wall helped, tall aspects raito tires hurt. Gives it that Cadillac flutter but feel that just doesn't feel right in a truck.