New Exhaust Problem-Maybe?

[Lowes Ride]

04' Silverado, 5.3, 4x4, Short Bed. I just had a local exhaust shop put on a cat back Flowmaster 40 series, dual, out the sides. Love the sound...Great rumble when your on it, and nice purr when on the highway....Anyways....the first couple of days, no issues, then I noticed that the rpm's seem to be a lot lower. At idle it is only about 500 to 600 rpm's. I don't remember what it was before the exhaust install, but it seems to be awful low. The second issue is that when driving, it seems to be going into a higher gear than normal. I always drive in the normal "D" setting, but when cruising at 45 to 50, it is only about 1200rpm. After a day or so of driving like this, the message center indicated "Check 4wd", or something along those lines... I shut the truck off, restarted and it went away. Any ideas????? I disconnected the battery, in an attempt to see if the truck can re-learn the new exhaust. Sorry about the long post, but..... Thanks for any help or suggestions!

[Rammer]

Everything sounds normal. These new generation V8's idle at 500 RPM. 1200 RPM for 45-50 is normal for 4th gear with the converter locked.