Just doing the math in my head. My midget runs 13s and was around 3500 rpms at 60mph (before and diff change or the 5speed overdrive installed). With 15s you should have much faster speeds with the same rpm and 3.90 diff. Anyways, there should be some play between the drive shaft and the wheels, that is just normal on all rearends. One quick check would be that the yoke nut is tight. I have seen this come loose on lots of makes and models of rwd cars and cause play and wired noises, etc. Usually takes a big socket to fit it and maybe a pipe wrench to hold the yoke to get the torque right. A set of diff gears in good oil should last almost forever without wearing out and whining or getting play or whatever. I'm sure you guys don't drag race these cars or drive them rough. I sure wouldn't if I had one.