I just picked up a 2001 2500HD 4x4 with the 6.0 and 4L80E. The truck has 84000 miles. Is there anything I should check out? Any good first modifications? It is going to be used for hauling/towing/plowing, so I don't need turbos or nitrous. Improved mileage/longevity is my main goal with additional power being second.