The 2005 year Cummins is the only year where the tow/haul button once pressed only alters the shift points in gears 1 thru 4 or raises the rpm shift points. It will not lock out 4th gear (overdrive) unless you purchase an aftermarket OD lock out switch/box. Don't know why Dodge engineers did this but they switched back to the other way in 06? To the OP, sounds like you are running in 3rd gear at 2500 rpm's unless you are running 4.56 gearing in 4th? With stock tires with 4.56 gearing in 4th gear you should be around 2300 rpm's at 65mph +/_ (can't find the gearing chart).