anyone know off hand what the head of the centerpin on a 92 xj would be? I have some shims from a fullsize ford I wanted to use under my g/f's jeep.

They should be 1/2 inch wide, 5/16th fine thread.

If the shims are steel you can drill out the hole to clear the spring bolt head and just weld them directly to the original spring perch. I think this way produces less stress on the spring bolt.