I've moved out to a higher altitude and I'm up over 5000ft above sea level. I was sure hoping the computer would compensate for this as my truck ran like a sports car in Arkansas but can't get out of it's own way here.
The BARO sensor and MAP sensor provide barometric information to the ECM to use in deciding how much fuel to insert in the engine. Lots of other inputs also contribute to the determination of the correct amount of fuel. They should adjust the information automatically when you go higher or lower in altitude and in particular the MAP sensor in the case of being under boost condition. This would be the place to start looking for a problem. Could be the sensors themselves or the circuits. I always think about the CTS sensor also. If it indicates the truck is cold when it's not, that greatly diminishes performance also. Will be interesting to see what the dealership finds.
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator