Can Zoloft be used to treat anxiety or is it strictly an antidepressant?
All the antidepressants can treat anxiety too as it and depression are produced by the same biological deficits. As a generalization, the SSRIs are actually slightly more effective for anxiety than depression.
What is the difference between it and Lexapro?
All the SSRIs do slightly different things, but are essentially the same, though this doesn't mean they will all necessarily work the same for everyone. Usually one or two are more effective for an individual than the others. Unfortunately, there is no way of predicting which one will suit you best.