Had Timbrens on my old '03 F250 and they worked really well. problem was, that over time, the factory springs seemed to have lost their oomph and thus the Timbrens were atop the axle all the time creating a harsh ride all the time. Contacted Timbrens and explained the situation and they sent me new ones (presumably shorter). However 2 things.1. after having original ones in place for a few years, the road salt made it virtually impossible to remove the nuts that hold them so would have had to cut/grind top off and start over. 2nd. got rid of truck while never having switched over to replacement ones.
All in all, I felt they were worth the price and results and lifetime warranty. Sorry this is under the air bag question,. but others have brought Timbrens up and shared their findings as well.