Changing gear ratio?

Hello Im a new owner of my first Tracker and have a ?? My Tracker is a 95 model w/ a5 spd trany. It is geared really low. I already have some pretty big tires on it and was wondering if it was a big job to change the ratio in the rear end and front (its a 4wd) What gear would I need to go w/ I dont have a clue whats in it from the factory but it is really humming at 50mph.
Thanks.