onegreatsteak.....
(hmmmm...im getting hungry just reading your name
)
... here is what (i think) i know about transmissions: do not get a truck with a Rockwell 9-speed; a pain to shift even for a pro driver.
i personaly would look for a Eaton/Fuller box:
a Autoshift, a Super10, a 13-speed or a straight10 (in that order
)
the rear axle is fairly easy to change as a complete unit; you can upgrade the axle ratio by swapping in a complete late model axle/suspension from a wrecking yard