Hey Ddiggler,
I'm an old Pennsy boy myself; caught the bobcat fever up on the Tangaskootack River way back when. What part you from?
We've got both jacks and cottontails here in Central Cali, and I've called gray fox with both. Which is surprising considering some of the blacktail jacks are nearly as big as fox hereabouts, several even looking as though they might easily whup a fox in a fair fight.
But I can report having MUCH better results on fox with high-pitched squeaky calls, rather than lower pitched gravelly-voiced jacks (which I often use to start a sequence for mountain lions). Something about a 15KHz+ Crit'R Call or a handsqueak that really drives grey fox nuts and they can't resist. So a jackrabbit call isn't my first choice for fox, or even bobcat for that matter. It will carry better in wind, though, and sometimes you have to play the hand you're dealt.
Incidentally, if you're talking about red fox, they seem somewhat more inclined towards fowl, especially waterfowl. Duck calls, if legal? The woodpecker calls would be another good choice here. Quail calls and the like would also be good coaxers. But I've called several Reds to within a few yards of me, with nothing more than lip and handsqueaks.
LionHo