I thought we had made it clear that with oversize tires your 3.73 axle ratio isn't quite enough gear to give the performance the engine is easily capable of providing. The reason the engine's not able to produce that power now is the 3.73 axle ratio is restricting its rpms. You were barking up the wrong tree with what you tried above. If you really want to be able to cruise at 75 without problem you need to regear your axles to a slightly lower ratio. 4.10 to 4.56 would do it for 31's. Or go with a smaller tire.
Again, your 42RLE's Overdrive ratio is lowering the engine rpms so much that the engine can't put out enough power to do what you want to do. Regearing will raise the rpms back up just enough to overcome the larger tire's added leverage against the engine.