Chevrolet Trailblazer Repair Help

Hello, My check engine light and now my ABS light in my truck won't go off. They've been on for months. I got a tune up last November and the engine light went a away for a few days, but has been on ever since. Also, I had my truck in 4 Wheel Drive for a little while a few weeks ago one day when it snowed really bad and ever since it seems like my car sounds louder when I drive.

I just bought a 2003 GMC ENVOY SLT w/ the 4.2L 6 cylinder.  I am having trouble w/ the A/C it will blow nice and cold on the drivers side but the passenger side and rear are blowing HOT air. I have been on quite a few forums and there are tons of people having this problem but nobody seems to have the answer. So if someone could help me out I would greatly appreciate it.