Trying to get 2 high flow cats 2 11" mufflers and an x pipe for true dual exhaust.
The exhaust shops keep telling me ill get check engine lights...
are they right or just trying to sell me their setup?

 

It depends. What year truck do you have? Any '97 - '08 it should work fine on. '97 - '05 4 cat system you need to have the rear O2 sensors mounted behind the hi-flow cats afterwards to keep the light off. '05.5 - '08 two cat systems its the same thing. The rear sensor mounted in the pipe gets mounted behind the hi-flow.