31 spline axels will be more important once you get your h/c/i installed. Right now it won't really matter much. But, of course, it's always easier and cheaper to take care of that now rather than do both in two separate steps. One reason to think about doing it all together is the ABS sensors. They get bound in pretty quick, and have to be smashed out if you ever need to pull the axels. That runs another $100 for replacement sensonrs.
At 77k miles, your bearings are probably in decent shape. Maybe you could look at it this way (especially if the h/c/i isn't happening for quite a while). Do the gears along for now. Then when you do the h/c/i, or shortly thereafter, do the 31 splines and rebuild the rear end with all new bearings.
BTW, the bearings that I was talking about are the 2 carrier bearings, and the inner and outer pinion bearing. Also, you can do the rear wheel bearings at the same time.