Thoughts on a wicked shake?

Ok so around 50-70mph and while decelerating I start getting a nasty body shake. It doesn't seem to be wheel shake no the whole front fenders and hood starting doing the paint can mixer dance! The car had a shake when I first got her but I initially attributed it to a flat spots on old tires. Since then I've installed a new steering box, tossed on different rims/tires (rebalanced) and had an alignment. This did not aid anything and in fact the shake got noticeably worse! Right now my Camaro is hogging my only lift but I plan on picking her up and taking a real good look around next week. Any ideas or tips on what to look out for? :/