Prepping an 8 3/4 Rear End...

Theres a drain plug on the side of the chuck but you prob won't get all the oil out. I guess you could try to pump it out. Unless you have seen it work I would pull the axles & diff, clean it out and inpect it. BCS you will only to replace a few gaskets. It will be alot easier with it out of the car than finding out theres something wrong after installing it. I bet theres a bunch of gunk you'll want to get out of there anyway.