OK, so after my impulsive whole-wheat-for-white flour substitution yielded super-dense bread, I poked around online a bit to find out how I could have gotten better results (besides the obvious solution of just not messing with my gram's recipe.) Here's some of what I learned:
If you're trying to do a straight substitution, you're going to need to add more liquid to the recipe, or you need to just use less whole wheat flour than white flour, because wheat absorbs liquid more than white. I figured this one out on my own, when 3/4 of the whole wheat was enough to absorb all of the liquid in the dough.
Whole wheat flour rises much more slowly than white flour, and you're going to want to let it rise at least 2x. Luckily my gram's recipe called for letting the loaves rise again after shaping them, or my bread would have been even denser and chewier.
You need to knead (teehee) whole wheat dough for longer than you knead white dough.
Doubling the yeast is a way to help whole wheat bread rise better/faster. I'm glad I decided to use the optional 2nd yeast packet in the recipe.
I found most of that, plus a couple more tips, on CliqueClack.
So, in summary:
-More liquid or less flour
-More rising time
-Less mixing before rising
Of course, all of this research wouldn't have been necessary if I had just followed the darn recipe in the first place.
At least I learned something.