Thanks for your response. He's 7, almost 8. In the mornings he's pretty eager for a walk, it's the afternoon/night walk that he resists most. In fact, we used to take him out late at night for last call, but had to give that up recently because he got so resistant to going. (I still feel weird about this because it seems like a long time for him to not pee, but he hasn't had any accidents and is still hard to get out at night when we try.) The growling and snapping has been building gradually for ... a while. I'm not exactly sure how long. Maybe a year? He's gotten multiple x-rays over the past year or so. At first the corn wasn't visible and we weren't sure what was causing the limp, so he was examined pretty extensively. There's never been any problem that showed up on the xrays, and the docs told us that if it was bone cancer that went untreated this long we would know it by now. So I guess we can at least provisionally rule that out. No one's found any fractures or any other condition yet either though. Things that we went through as possibilities include: arthritis, shoulder injury, bone cancer, back injury, and one animal surgeon suggested just amputating the toe without us even knowing what the problem was! (We refused.)
He does typically let us manipulate his foot when he's standing, and touch the corn. Not when he's lying down, unless I"m putting musher's secret on his paws (because he knows he always gets tons of treats when I do that). I think there was one other occasion when the foot was really painful and he growled at us for touching it. I honestly can't remember what it was, maybe after one of hte times it was hulled? But it passed very quickly.