Lincoln:
Tetanus injections seem to be the choice of injection for one's rear end
It just so happens I am overdue for one of those, but it will probably be the same practice nurse who did it last time (in my arm), and she'll probably remember I didn't have a needle phobia then. Besides, I can't imagine saying I had an arm and thigh phobia but not a buttock phobia would be very convincing. Still, thanks for all the ideas, everyone.