when's the last time you had a tune up - plugs, wires, cap, rotor? could be the ignition system needs to be freshened up.
I have a v1500, 350, 4l60 that sits on tires that are also about 32in. it motors down the highway fine at 75mph, but isn't winning any land speed races. downshifing on hills is expected, and not that different than my 2004 suburban. the biggest nemesis on the highway is wind. driving in high winds (central ks, Oklahoma, etc) just kills it way more than it does a more aerodynamic vehicle with a modern drive train.
I use D around town, and OD only on the highway steady above 55mph. I don't tow much, but when I do, it is D only and you definitely have to adjust your expectations on speed....ain't no 75mph towing going on...