suspension...
Find out how the truck was dropped...
A garage should not have to charge you if you need them to put it on a lift for you.
Your truck was probably dropped with replacement springs ( or cut springs ) in the back and torsion keys in the front.
Some people save money by leaving the stock shocks in. Stock size shocks may cause some of the "banging" you describe ( the shaft compresses inside the shock body to it's limit )
Replacing the shocks with new ones may have given you a better ride temporarily, unless you installed "lowered" or shortened height shocks.
The banging noise needs to be addressed. A complete visual inspection of the undercarriage should be performed. Again, a shop should at least help you check for any damage and make more suggestions...