i still don't understand about your obsession with a degree. does a degree helps to contribute more cash flow or savings to your company? if no, then you have no bargaining chip.

If having a degree does helps to put in the extra knowledge gained and channel another say 15% of profits to the company, then maybe you can ask for 10% of commissions or increment. Otherwise, moving on to another job has its same effects - another company might be looking for a degree holder because he/she already learned the fundamentals/theories, or because the position requires so and they can afford it in order to fill a vacancy. what i'm trying to say it, the new job might have higher workload, new learning curves or new workflow in return for a small income increment.

And i don't think u're underpaid at all.