I just want her to be friends with one dog!

I'd love to say "this will all work out" but having 2 females of my own (a lab and lab-mix), I can tell you that it may not or at least not the way you'd like it.  My lab, Sassy, was also a rescue with fear aggression.  The 2 do okay around each other most of the time, but the one thing that would never, ever work is to have them confined in close quarters.  They have more of a tolerance for each other much of the time and some of their fights have resulted from just being too close with no where to get away.  I can tell you that with Sassy and her fear aggression, putting a muzzle on her and putting her into an uncomfortable situation would just make things worse for the relationship and pretty horrible for her.  I'd certainly do it if it was absolutely necessary, but not as a way of getting the two to learn to like each other.  Even having one confined to the crate while the other is out may be creating some jealousy between them.  I know how difficult this can be, so I really sympathize.  Maybe someone else will be along who can offer some advice.

I am sorry I don't have first hand advice for you - but I would recommend a good trainer or behaviorist (who works with dogs on dog-dog agression).  I know ours does and the way he goes about it is using his trained dogs, or dogs whose personalities he knows and who he is working with, to work with the agressive dog.  I only know of this because he has used our dog to work with one or two dog-agressive dogs and had asked our permission to do so...  I think that a professional, who will be able to read both dogs' body language is the best bet, because based on an analysis of what is really going on with each dog, he or she will best be able to give you suggestions on how to proceed. 
 
I also hate to say this, because I really believe there are no "problem breeds" there are problem people who breed for the wrong reasons or who don't socialize their dogs early and often - but it is my understanding that pits sometimes can have problems with agression toward other dogs, so you may have an uphill battle on your hands.  I do hope some trainers and behaviorists, or someone who has more experience comes along and has some practical advice for you. 

It is true pitties can have problems with other dogs.. sometimes it doesn't even show up until they are around 2 years old and have been fine with other dogs their whole life up to that point. It's sad.. but sometimes it's something those of us who share our love with this great breed have to face. It has nothing to do with how their raised or anything you have done wrong.. it's the breed.. so don't feel like you failed in any way if it doesn't work out. Some of them go through life never showing aggression toward another dog.. others would just as soon take one down as look at it.

I have been fortunate myself that Boss is doing just the opposite of that and getting friendlier with other dogs as he gets older.. he's a weirdo! Now if only I could get him past his aggression towards stange people!

Putting the muzzle on her when she already feels uncomfortable could be adding to your girls aggression. They are left without a way to defend themselves and can sometimes act more aggressive, or do what she is doing.. hiding behind you waiting for you to protect her.

I hate to say it, but seems that many dogs dislike Boxers because their body language and play style is different.  Boxers seem to play well with rough and tumble Labs, and other Boxers, but often have trouble finding doggy friends because they are sometimes perceived as rude by other dogs.  [sm=lol.gif]
 

I would seek a professional's guidance to be honest with you.  Maybe these two can coexist, maybe they can't but if she is more scared with the muzzle on it is likely doing more harm than good.
 
Sorry I couldn't be of more help.  This is a board that specializes in pit bulls.  There are some people on there that know a great deal about the breed www.pitbullforum.com