Hi - sorry no-one replied earlier (I've been away so haven't had the chance to post as much as normal!!!)
We have a similar problem with cats, so I'll run through how we deal with this in the hopes it helps...
The key is to try to break the association between trigger (squirrels) and behaviour (squealing).
It is definately easiest to acheive using a clicker - if you haven't done any clicker training training with her before either get a clicker and introduce her to it at home, or introduce a suitable cue word that can substitute for the click (nice, or yes are often used). Make sure she associates the click (or word) with yummy treats...
When she sees a squirrel try and click (and treat) BEFORE she makes a noise - just in that instance that she has noticed the squirrel... this is the best way, as you don't need to try and calm her down to work with her. Then every time she turns back to the squirrel, click again (and treat) - you will quickly find that she looks to the squirrel and then starts turning back to you in anticipation of the click... you have your first step achieved!!!
You will need to repeat this many times with many squirrels before she learns that it applies to all squirrels, as dogs don't generalise very well, but once she is calm on first view, you can start walking closer to them, and again clicking for looking at them... soon the sight of a squirrel will just be a cue that she gets a treat from you.
I wouldn't the treats out for this for a long time, as squirrels are very rewarding, so the behaviour of not reacting will need to be seriously ingrained before you fade the treats... but it is possible to do - just stop clicking every single look, and reward with a simple "good girl", then click the next look, and fade away that way...
If she has started making a noise before you get the click in, turn and walk away from the squirrel - get to the point where she can listen to you, and ask her to sit - click and treat this sit... she will then (assuming the squirrel is still there!!!!) turn to the squirrel, but you will be expecting it and will be able to get your click in before she reacts....

Hope this helps!