Starter spinning very slow and car won't start.
 

I've got a 2001 blazer 4.3 and it's having some issues. The starter died and we replaced it with a brand new one from Autozone. We put a brand new battery in it as well. It still spins over very slowly and won't start. It sounds like it's losing power to the starter and doesn't have enough juice to spin fast enough to start. We checked voltage of the battery and it was good. We checked voltage while the car was trying to start and it dropped down to between 9-10. We did the same test on another blazer we have and while trying to start voltage only dropped to 11.5. So it seems we're losing power to the starter somehow. We also ran a wire from the positive on the battery to the 12v source on the starter to see if there was something draining power from somewhere else but it still spun very slow. The car has oil and the cylinder walls are oiled up and everything as well.

We don't really know where to go from here but we need this car running very soon. Is there anything you guys can tell me as to what to check next? Thanks for the help.

Zach