The truck should have round wheel bearings which if remember right, adjust a little different then the modern tapered wheel bearings we are used to. Not knowing the tire size, it could be normal to have just a very slight movement in the wheel bearings, the bigger the tires, the more you can safely have. Just for reference, in Missouri a 11.0 R/22.5 tire and wheel can have up to 1/2" movement at the outer most edge of the tire before DOT will reject it.
Tighten the bearings up snug so you know you have zero play, then check for kingpin movement.
If you have vertical movement, like straight up and down, then the rebuilder left out the shims in the king pins. These shims go between the axle and the top spindle bushing.
If you have slight in & out movement at the top and bottom of the tire ( with bearings snugged up tight ) then you are dealing with spindle bushing play or king pin to axle bore play, both of which should have been dealt with when the axle was rebuilt.
Try and get a better idea where the movement is coming from and get back to us.
Joe
