How can I increase income to pay student loan debt?

Drive the trucks instead of dispatching them?

If you already have customer service and engineering experience, and are exploring IT, I would say you should be able to start out as a customer service/first tier support person. (Not sure what JLPT J3 is, specifically, but if it is everyday Japanese, you should be able to exploit that as well, probably of more use for international organizations with locations in multiple sites - not necessarily staffed locations; I'm thinking of companies with datacenters in remote locations where they don't have offices, but having someone who speaks the local language could be helpful.) What you can find depends on where you are -- are you in a major metro area, or willing/able to move? Also, if your local community college offers courses in networking that could lead to a solid certification like http://www.cisco.com/web/learning/certifications/associate/ccna/index.html, that may be a good track -- you obviously wouldn't want to take on more debt, but if two semesters at like $350 a pop could get you the paper to increase your earnings, it might be worthwhile. A CCNA with customer service experience should be worth at least a second tier support gig in a company that supports real networks. With audio experience you may be able to find something in a media specific company as well.

Teach in the Middle East. Not the greatest lifestyle but you can save a bit of money. To really make a difference, work in a high school that will pay for your masters and then work in top tier international schools. Some people are able to save as much as thirty thousand a year. Hard-core savers only, but it's still possible. Don't bother if you don't enjoy teaching. I just wanted to point out that there are routes to getting a degree and career-changing that don't cost you any money.

Video game audio editing is tedious as hell, but if you can get into that racket, you can easily go from $15 an hour to $25 in one step. Any game companies near you? Any studios that get subcontracted to do game audio editing? Sports games, in particular, are very editing intensive, because there's so much sportscaster dialogue.

I hope you're already on income based repayment. Also remember that if you work for a non profit, some of your loan loan debt can be forgiven.

What about instructional design, specializing in creating computer-based training courses? If you picked up some software skills to creating training courses, and you read a few books on instructional design, you could propose that your degree helps you with that career. Plus, you have training experience. If you approached a business that has lots of warehouse workers, truckers, etc., but also a big office presence, I could see that they would be really interested in your skillset. Are there businesses like that where you are?

As a Japanese speaker, I can tell you that N3 is not sufficient for a job in which you will be paid for knowing Japanese. It's an admirable achievement for learners, but it translates to reading at about a 4th-grade level and while it does not test oral skills at all, the N3 level suggests the student isn't ready for everyday business communications with native speakers. I think jobs where Japanese is a relevant skill require something beyond the N1 level in terms of proficiency. If you wanted to spend a few years of study to bring up your Japanese to proficiency in all the joyo kanji (including the commensurate growth in vocabulary), you could see about breaking into freelance translating. I know a few in that field and they seem to be doing quite well. You do have to consider if you have the temperament for it, though. It is a lot of time by yourself in front of a screen. You could always go back to TEFL. The Japanese market is pretty bad, though, from most accounts that I am reading. I occasionally browse the job listings out of nostalgia and 250,000 yen/month is a pretty common salary, which is less than you are making now. I think it is also worth noting that at least in Japan, TEFL is not a long-term career unless you can get a position at one of the few universities that extends tenure to non-Japanese. Things might be better in China, Korea, and other locations, but I don't know about those places. Given your dispatching background, you may want to make the jump to driving the trucks as oceanjesse suggests.

Telecom or anything involving signal processing should be your focus if you want to make more money and not waste your degree completely. Start seeing what the job market is like where you are and get whatever certs you need to get your foot in the door in those fields.

If you have a clean driving record and criminal background and can find the time get yourself a CDL, there are jobs out there that pay $300 a day. Now that's for driving 600 miles that day, but hey. Most trucking companies are understaffed, and there's no shortage of work to be done.