Main culprit for this 55 MPH Speed Limit = Onset of aftertreatment problems. There are symptoms, causes, and fault codes that you need to watch out for if you are limited to 55 MPH and what you need to watch out for to prevent your truck from entering a 5 MPH derate. 

DPF filters are typically made of a ceramic honeycomb-style substrate that can filter out tiny soot particles without creating a large restriction in the overall exhaust flow. The filter will trap soot particles till it reaches capacity and the restriction level becomes too high. The filter then goes through a process called regeneration or regen.