hey, guys. similiar thing happened to my truck.

my rear driveshaft was connected and everything was good to go, visually...but, once i torn it out, i saw that the screw that connects the rear diff to the shaft was loose. all i did was tighten the screw with a hex key.

what is that screw called? it's the one that has the thread outside only, no head, no anything. just a cyclinder with threads..but if you look inside, it has a hex shape grove...what is it called?

thanks,
trung