Agree with the numbers neutral at 2850 and gear at 2600. Mine had the same issue, I called american diesel and brian confirmed these numbers.
If you don't reach 2850 in neutral, it's because somebody dialed your limit screw back to prevent it from reaching that rpm. Unnecessary, but doesn't hurt the engine. If you do reach 2850 in neutral and don't reach 2600 in gear, it means you're over-propped. According to brian, bad for the engine - like leaving your car struggle in high gear as you're going up a hill.