I just bought the truck and I'm having problems too with the 2-3 shift. At around 30mph it won't pick 2nd or 3rd. It just shifts back and forth (hunts). I'm getting vacuum to the modulator, I think I'll replace it and see if it fixes it.
Today was the 1st day I've really driven it a bit. It also seems I have only 2 gears. I think I only have 2nd and 3rd gears.