[xlr8or]

That early of an SPF should have the typical 8.8 T-Bird 28 spline differential. The 1/4 inch seems excessive like maybe its not actually seated in the diff. If you take the rear control arm loose enough to be able to pry the axle stub out of the diff (you will need to do that anyway to replace the boot) there should be a retainer clip near the end of the splines and it should go in with a definite click. If the axle is shot you can problably get a replacement off the shelf Hillbank. There is also some aftermarket axles being done with better CV joints but I can't remember the name of the shop.. Check the SPF forum and someone there will know who it is.