all of the 8.75" resr ends I have around here use a pair of 3/8"-16 bolts to secure pinion snubber. Yours may have the threads full of dirt or corrosion. Try cleaning them out with a tap, if that is the case.
Yup, he is right. Because the bolt holes are in a vertical position, the holes fill up with dirt or dirt daubers build nests in them pluging them up. Run a 3/8 th x 16 tap thru them and blow them out with air.
