I have a similar problem with one of mine, he was being picked on by some of the other males. I moved him in to own aviary, which he seems relived at. He spent so much time in his nest box that once I even had to open it to check he was still alive. He's about the same age as squeak, he comes out a bit more now, but apart from eating he just sits near the top of the cage watching the others mostly. He can be handled, but will often move back in to his nest when I put fresh food in. I still can't work out chipmunks and how they behave. Most of mine are like your zippy, but a couple are very quite, but tucker is the most quite. I'm sure he is less stressed having an aviary to himself, but I don't think you should split yours up. I have groups that have always been together since birth, and throughout the day they will have sqaubles, but despite the odd argument they always end up in the same nest box together at night, even though they have a choice. I'm thinking about introducing one of the quieter females to tuckers aviary soon, in the hope it will make his life more interesting, I don't think he is getting enough excersise. I've even left the door of his aviary open to see if he wants to come out, but he's not interested yet. He hasn't got a wheel in his home yet, so I'll try that. Even though mine have loads of space to run around most chipmunks are almost obsessive about running on the wheels. I've found the best type of wheel is a Wobest size senior, which I get from here
http://www.petplanet.co.uk/product.asp?dept_id=65&pf_id=2615 This type of wheel is one of the rare ones which can't chop off their tails as their running. The other thing I'm going to try for Tucker is to make his home even more interesting by putting in more tunnels and tubes. Although that will mean I will probably see less of him it might help his confidence.
I don't know what your nesting arrangement is for squeak and zippy. I try to ensure mine can have one each, I also read once that their nest boxes should all be on the same level, something about dominance if one is higher than the other.
One thing to add, to begin with I always thought mine would be missing their parents when young, so could they be sad at being split up. I've learnt after several litters that once they have been weaned, their mother will have hardly anything to do with them. Most will even move to a different nest box, it's not what I expected at all. I've even found that brothers and sisters have needed to be split up as they get older because of fighting. I will never be able to work out who is going to get on with who, and it seems to change over time.
