I agree with the people that have already advised you to continue working towards your BS. I have been working full time in the Aerospace industry for two years now (I graduated with a BS in ME in Dec. 2004), and at my company, I see many opportunities for career advancement for those with a degree. Of course those without the BS degree also are eligible to apply for the positions, but the positions generally require many, many years of experience in lieu of the degree. If you were to take the job and only achieve the associates, you would likely be stuck in the same position without much room for advancement for at least 5 years.
Also, you may begin working as a designer and find out that after several years of it, it is no longer exciting. Once the work no longer interests you or keeps you from being bored, you will need to search for a different job. If that happens, then you will probably want to return to school to obtain the BS, but this time you'll have more bills to pay and possibly even a family to support.
Another point to be made, although unfortunate, is that in today's world it seems that the best way to advance your career is to change positions every few years. Having a mechanical engineering degree will definitely help you to secure different positions early in your career while you are still making a name for yourself.
Finally, the college years are great years! I miss being in school and almost wish that I hadn't worked so hard to get out soon. I'm envious of my friends that are still working on their degrees, because they are still living the fun college life. Don't short change yourself on some good experiences just to make some money. The money will come eventually, and money does not equal happiness!
Good luck on that degree!