Sounds like a good ol 3/4 clutch failure...
Yes, drop it into 3rd. As far as avoiding downshifts, learn what speeds the truck shifts into 2nd and 3rd at. Then when you want to hammer down, drop it into the right gear for the speed you're going, put the pedal through the floor aka WOT, then put the shifter into 3rd so the computer can do the shifting.
