marshallkarp:So, I would guess, to put the wheelez beach tires on, you take off the wheels, run the axle through and put the wheelez wheels on?
Longer axles and spacers as crpntr133 said.
Axle rod stock can be found in good hardware stores, but Wheelez carries axles that are pre-machined:
http://www.wheeleez.com/axles.php
Dave C