I have a 2006 3500 4 whd with the automatic, the tow haul button really hepls out here where the hills are long and tall when towing one of my trailers, the car trailer is the lightest. right at 8600 lbs on the axles, the trans won't shift into O.D., or maybe it is locking up the converter , until after 55 MPH, that is with the 3.73 gears. It will downshift automatically when I'm off the throttle going downhill when it gets to 5 MPH above where I have the crusie control set at, usually close to 65 MPH My 1998.5 didn't have that feature.I like and use it all the time when towing Yours should have a lock out O.D. on that button also, two clicks instead of one