2005 GMC Sierra Hard left turn

The only way to solve this problem is to have your vehicle scanned at a dealer. Your powersteering has electronic controls on it that vary the PSI of the powersteering fluid to the gear box - as well as various other sensors. Dealers are usually the only place to look for the proper scan tool..
The EVO (electronic variable orfice) controls fluid pressure - the SWP (steering wheel position) sensor - the VSS (vehicle speed sensor) - the PSCM (power steering control module) and the VCM (vehicle control module) should be scanned.