If you would be able to post some pics of the Blazer, we might be able to answer this question a little better. The PO (previous owner) might have just cut the springs, and not added shock relocators can cause it to slam into the tub when you hit a dip in the road. A bagged suspension would be a great addition to the Blazer, if you have the money. If you're looking to correct this problem cheaply, I would consider picking up some new rear springs and shock relocators from a vendor such as
ECE (Early Classic Enterprises) or
CPP (Classic Performance Products.
Good luck with the build!