When we went to Moab a few weeks ago from Florida, I was having the same issue and made our group aware of it. Around 68-70 on even the slightest of grades the engine didn't have enough power to get up without turning off the overdrive. I found the sweet spot on the way home from Moab at around 78mph on gps not on the gauge. Back here at home i'm not having any issues at all, it was just on the long grades on the trip. I didn't use cruise control at all (I've never liked how it floors it to maintain speed) and i'm running 32's also.