Why is my truck in limp mode

Sounds like a concern with the engine performance and possible transmission systems. Hands on diagnostic testing is required -- there will be diagnostic trouble codes (DTC's) set -- so you'll want to drill down and find out what's going on starting there. No way to guess without testing. You may want to enlist the assistance of some local techs near you. Here's a handy link to search for some local shops -- www.repairpal.com/repair-shops