ahh, good old chunky ujoint
. There I'm thinking of pointing it at the inner corners of the yoke, i.e. lust left (forward) of the jbweld, pointing at the centerline of the yoke.
also putting longer ujoint bolts in and clamping some two sheet metal "paddles" on would be easy, and point the sensor forward so it only sees the paddles as they go by would make for a well balanced signal with more even on and off time.
Little plumbers tape, tightened with a machine screw, to hold a machine screw threads out, might be all you need for bracketing on that diff.