Standing on the side of the truck, I grab the rear wheel and push/pull the truck side to side and it clunks in the axle.
Sounds like the shafts move in and out a little bit.

Normal?

 

Yes, completly. C clip axles do that, just some normal lateral play in the shafts

 

Is the truck in the air when you do this? I ask because they shouldn't move on the ground...but wheels in the air, yeah, there's some lateral movement.

 

They do, it freaked me out with my first Ranger, but that was about 6yrs and 110k ago. New FX4 does the same thing. A little clunk when shoving the rear back and forth. C clips have just enough play to seem "wrong".