traxxas isn't exactly known for there precision so... where exactly is the play coming from? the wheel itself? turnbuckles/ball cups? plastic parts just sloppy on hing pins or screws?
if the wheel nuts are tightened properly, the wheel bearings are good, the ball cups are good (does the slash have turnbuckles and ball cups or does it have those dumbass fixed links?) and the wheels aren't bent out of round you're stuck with it. that being said balancing the wheels may help a bit.