Okay, so I talked to the guys at eTrailer and they said that they were not aware of any trac-bars that work with an air-bag equipped E450. Similarly, the guys at hitchfinder.com sent me this message:
I’m sorry but I am not aware of a Track Bar that will work with air bags.
We are an independent online dealer for Blue Ox products, you may want to check with Blue Ox directly to see if any modifications can be made to install a Track Bar. You can contact them at www.blueox.com <> or 800-228-9289.
Thank you.
Les Narum
Customer Service Representative
Hitchfinder.com stores >
However, John Henderson at SuperSteer was short and sweet:
Andrew, the SS450 trac bar will work with rear air bags.
Thank you.
So, clearly it depends on the brand. I have a few emails/calls in to other vendors but for now my money is on SuperSteer. Plus, they were very polite and professional on the phone. That's a plus in my book.
EDITED for bad typing