Creepy. Where's the shudder emoticon when you need it?
Anyway, I do think Jack is out of line. However, I don't think you should "warn" Jill about it. My main concern is that if you tell Jill about it and "tattle" on Jack, he can easily use it as ammo if his mission is in fact to isolate her. He can say that you are lying and trying to sabotage the relationship and make him look bad. Basically, I don't think you want to put her in a position where she might be confronting him.
I like the suggestion of asking Jill whether you're leaning on her too much, especially if you know that you are looking for her support a lot more than usual. We all go through tough times, and friends and family are there for us (just like we'll be there for them). I don't think there's anything wrong with checking in with Jill in a "I just want to make sure that I'm not distracting you from the other important relationships in your life" kind of conversation. If she says no, well, great. If she says yes, you two can adjust as needed.
As for Jack putting the pressure on you, I think you need to remind him that your relationship with Jill is different from his relationship with Jill. If he has concerns about his relationship with Jill (the amount of time they spend together, living together or not, etc), he needs to bring them up with Jill. You will not try to get Jill to do what he wants to do, and you're not comfortable listening to his venting, as you two aren't close. If he wants to vent, he can talk to his mom or one of his guy friends. Tell him to encourage Jill to bring up her relationship concerns with you if she has them. "Jack, Jill and I have been friends forever. I hope that she feels comfortable bringing it up with me if she has an issue with our relationship. If she does and she isn't telling me, please encourage her to bring it up with me directly."