About 6 months ago I took my 2007 E350 w/sport pkg, 18" staggered wheels into the dealer to have the suspension checked while it was under warranty. They found the suspension to be solid but noticed a crack in my front right rim. I had it fixed at my indy and decided to put new rubber all around.

Ever since I am getting an extremely slight vibration, in fact, I can only really notice it when nobody is in the passenger seat. When I hit about 55mph the seat back shakes side to side (door to door direction).

There is no vibration in the steering wheel or any I can feel in my drivers seat. I'm thinking maybe the new tires need to be road force balanced. Especially the rears since there's no vibration in the steering wheel. What do you think?

Today I pulled each tire off and cleaned them real good and checked for cracks or any signs of bending, I couldn't find any.

 

You are on the right track. Our E also had deformed 18" OE wheels, in fact, all four of them. We replaced the whole set with great aftermarket wheels. Force matching, possibly a new wheel, may be required, but it does seem the root cause is one of the rear wheel/tire assemblies based on your symptoms. See if the shop which does the road force balancing will let you watch and you'll probably see any remaining out of round wheel condition while it's spinning.