need help! I don't know nothin' about gears!

If you are going to the expense of changing the rear use a 8.75 because it will allow you to changes gear ratios easy and there are more parts and options available.
I paid $250.00 for the bare housing for my duster.
$333.00 for axles from Dr. Diff.
Re-used the stock brakes from the rear end under your car.
Around $500.00 or so for a used sure-grip rear with the ratio you want.