Level of difficulty changing 8 3/4 gearing

If you truly have an 8 3/4 (your car would have come with an 8 1/4), changing the 3rd member is fairly easy. Changing the gears in your 3rd member takes a lot more knowledge and expertise. As mentioned, it would be a lot easier and probably cheaper to just swap out the whole 3rd member with one that has the gears you want. Freeway gears should be easy to find. An added benefit to that is you'll have an extra 3rd member for performance use or emergency repair. If you don't need a sure grip, I would have a 2.76, 2.93/4, and probably a 3.23 with the small (7260 2 1/8") u-joint. You're about 1 1/2 hours north of me.