In high mode on my MiniMaxx, my F-450 is already in fith gear w/ torque lock by 40 mph....regardless of my load. I suspect if I hammered it in tow/haul, it may shift a little better, but yesterday, pulling 26K lbs of hay, 2800 lb trailer...decided to manually shift. Only way my tranny wouldn't shift quickly into 5 t/lock...lugging the heck out of my motor. Exhaust temps rose quickly also. Had to remove the bra. Coolant temps up to the point of "defueling".
Anyone have the same prob OR have you corrected it somehow...well....short of a different tranny

))???? Thx. '08 F-450, CC, 4X4, LB, 4:88's