Do not return to that shop - no credible shop will spray wd-40 to solve anything. The wd stands for "water displacement" and that is the intended purpose of the product - to displace water.
If you were hit at high speed, much more than your muffler should have been replaced. Its more likely that the entire exhaust system should have been replaced. It doesn't take much of a rear impact to kink the exhaust at the front.
You have a unibody vehicle with no frame. The alignment and suspension need to be checked right away by a qualified shop and you really should have the bodywork inspected by a reputable shop.