If you buy a pinion cup, buy a driveshaft as well. They need to be changed together.
So if I have a low milage ds that looks good, it wouldn't work? Well it may work, just not correctly? ???
If the pinion cup is worn, the driveshaft that was in it is equally worn.
If you replace one, then the other will very shortly be worn just as bad.
But, yeah, if you had a good driveshaft sitting around, a new pinion cup is fine...just examine that DS carefully for wear before using it.