Have you looked to see if a better geared center section for your rear axle is available? Or swap an entire rear axle.
That would be my first choice, and only then an auxiliary transmission if more or faster gears are still needed.
If I'm not mistaken a Mitchell overdrive is not very heavy duty? That said if you have one and it is in good shape it could be useful assuming it stays together in your 2 ton truck with plenty of torque.
Grigg
