Each gear in your transmission has a gear ratio. Besides that, you have a secondary ratio that all of those gears get put to the ground through, which is the ratio of your rear differential.
Keep in mind, though, that you're driving the front wheels too, so your secondary gear ratio needs to be changed in the front as well as the back, so they are the same.
So, if you're looking to change the secondary gear ratio by swapping out the rear differential, you also have to open up the tranny and change that (front) differential as well.
Hopefully that makes it clearer.
EDIT: Go to the VFAQ and in the Drivetrain section under the Manual Transmission heading there is a "GEAR RATIOS" VFAQ that might help you visualize it.
EDIT2: TRE used to sell 10%, 15% and 25% taller 5th gear replacements. I can't seem to find any of them on their website anymore though. No idea which application they came out of, though.