Gusstone,
i ran across some material i have on this truck and there is actually a segment in the troubleshooting section of steering that actually 'specifically' mentions "Vehicle wobbling - straight line instability".
Possible causes:
-Malfunctioning PS valve
-Worn or inappropriately adjusted wheel bearing
-loose road wheel
-excessive freeplay of tie rod clevis pin
-deformed road wheel
Then there is "Excessive freeplay of steering wheel"
Possible causes:
-worn steering shaft and PS valve-connecting spline
-loose PS valve mounting bolt
-inappropriately adjusted wheel bearing
And finally "inappropriate steering wheel return travel"
Possible causes:
-Malfunctioning spool
-Inappropriate centering spring of PS valve
-Inappropriate wheel alignment
-Insufficent tire air pressure

These are the possible causes of wandering steering as you described it.
the possible causes are things to look for so it's time to go fishing sir ;o)
If you've rebuilt the steer axle then you can rule out anything in that area (with the exception of the steer cylinder as previously mentioned) you should check that too.
I also was looking in a diagram of the steering system and it references a '**** position control system'. Does this truck have that? a sensor on the steering wheel and a sensor on the wheel angle? It mentions in this system something about a '**** position correctly solenoid valve'. It describes "By sending some operation oil to cylinder piping by the opposite side, many handles are rotated to a motion of a tire." "Rotation of a handle catches up with the motion of a tire".
But then again if you have play in the steering linkage or a loose wheel that could also contribute to auto correction based on that system causing the truck to wander while trying to drive in a straight line.
Just some things to consider while troubleshooting :o).
"EDIT"
Oh i just discovered FLA's AI caught a word it didn't like... apparently 'K n o b' is an unfavorable word in this forum? Well thats the word the service material uses so it is not being used in any defamatory way but i digress, it is what it is.