I am wondering if these are required for just a SPOA.  Assuming that the rear driveline is the same comparison as the front driveline.  I took some measurements on my front shaft and adding a 3/4" shim would only leave 5/8 inch of exposed spline on the front driveshaft.  I don't feel that is enough to compensate for axle compression.  Any comment or opinions??

I don't run one on my rear driveline, and I didn't on the front drive-line until I switched to an RUF conversion.  The only time the drive-line has dropped out was when I sheered off the axle centering bolt off the spring and slide it forward a few inches.