If you have the 68rfe, it shifts really early, bogs down, and ends up working g the tranny very hard. Get some transmission tuning with EFI live to up your line pressure and shift points. Otherwise, don't let it go under 1400rpms or so if you are trying to accelerate or go up hills. Mechanically, I'd stay at 2000 done all day rather than be at 1400 when towing with 7k+ lbs. The 5th and 6th gear part of the tranny goes first, and hitting it down making power doesn't help it.