This is a 4.3 CPI W series
I was driving on highway when engine light came on, took exit, upon stopping very rough idle, terrible exhaust fumes, no smoke, poor brake pedal.
Used Seafoam with fillup gas, ran w/no changes. This engine has run flawlessly til now. Fuel? Spark? Most likely?

 

What year? Did you have the DTC's retrieved? What DTC's are in memory? Sounds like the EGR valve may have stuck open. Depending on what year, there could be other things.