In my opinion, shocks wear a whole lot faster than springs sag.
Replacing shocks at about 40,000 miles is pretty normal, but many wait as you are, to do their suspension work. Also, your bushings, and TRE's, Track Bar, and steering dampener are wearing out sooner than the springs!!
I would replace the shocks with cheap walmart type until you are ready to do your lift, and replace with Rancho, Bilstein, OME, Pro-Comp, or whom ever you feel will give you the best ride, and shock life. Shock replacement may even improve your handling and ride!! Replacing your worn bushings, and such is a better idea.