How long have you had the truck... another possibility is the little arm on the shifter isn't engaging the gear.. Mine is doing that right now, but since I don't need the speed, its not a problem.. the reason for my problem is the lip on 2nd gear is worn smooth so the arm misses it.. I have many more gears, just not in the mood to change it right now...
You said something interesting.. It will shift when you hold it up... leads me to believe that your problem is tuning related or maybe...... running on grass... even with my engine running great.. it would rarely shift in the grass... even short grass and you are only running a short 100 feet...