Jeff, you might need a code reader to figure out what is wrong with the exhaust brake on your truck. Usually but not always a DCT code will be stored in the ECM on your truck that may tell you what the issues is.
If the Actuator is falling or has failed, you may need to install a new turbo actuator as John in his post above has stated.
Geno's Garage sells a new turbo actuator from City Diesel that is a cost-effective way of repairing the turbo. Without replacing the entire turbo charger as a dealer would do.