[randy500]

Straight axle chevys wandering is always due to frame flex or cracks.
The steering box mounting area is frequently cracked and usually not readily visible.
The front crossmember rivets are almost always loose.

Inspection for cracks: 2 person job, one person moves wheel left snd right while another uses strong lighting to inspect the frame inner and outer around the steering box looking for cracks to open up.

Inspection of rivets: 2 person job, one person shakes wheel left and right the other person holds the rivet and then the crossmember and frame, the rivet should not move nor the frame and crossmember. Repair with bolts that fit the holes very tight, rivet replacement procedure is in the chassis manual i think if you want to look it up.

With a stiff frame the wheels transmit their force through the tie rod to the steering box which restrains the wheel from moving which causes the darting around.
A cracked frame and or loose rivets doesn't restrain the wheel and the truck darts around.
These things are never found via alignment.
Front end mechanics dont look for this stuff.

Off road design has a steering box frame reinforcement and a bracket that ties the drivers frame rail to the crossmember which further stiffens up the front frame.

Loose or missing engine crossmember bolts are also frequently found too. The engine crossmember also ties the front frame rails together.