How can I balance an ESL classroom when I have an imbalance of high students and low students in regards to level of English?

If they are wildly different levels, you can't really do it. You must split the class into two and teach two lessons--give the one some busy work while teaching the other. And do it in tandem.

If the differences are relatively minor then there are a variety of techniques:

• teach to the median level of the class
• pair strong students with strong, weak students with weak
• sometimes pair  weak and strong students together: observe which strong students help the weaker one rather than getting impatient or frustrated. Figure out how to use these 'sub-teachers' as helpers.
• Make the weaker students feel like a valuable part of the class(are they funny? are they clever?)
• Pair yourself with the weak students some time if the others are bored.
• Don't expect as much from the weak students. Be happy if they do something within their level.

I've taught in classes in elementary schools where there were several different levels at once.

At that point I followed the syllabus/textbook and just got through it.

Sometimes teaching is a joy. Sometimes it's a job. 

Either way, they're paying you, so you get through it!