Not really much of an answer, however most Sierra's and Silverados have an issue with the XMSN "popping" or slipping from 1st to 2nd gear. Last I checked there was not a recall for the XMSN's for that model year.
As far as the ABS lights, check to see if you can get the OBD scanned and reset the codes (any auto part store does that for free) and drive for a few days to see if it will come on.
The mileage could be due to your rear axel ratio. If you have a truck with a 4.10 ratio, it'll feel more sluggish that a normal gear ratio of 3.73. Copy you VIN down and have the local dealership run it for you. They should be able to get you a list of the standard options that the truck had from the factory. Otherwise, the prior owner may have put in a 4.10 axel for haul/towing/pulling.
Hope that helps!