Thanks for all the responses! For some added clarity:
This was the first investment property I purchased, and now own a few others which are also cash flowing. Obviously, I'm getting a great return on this, and the interest rate is locked at 3.25%. There are a few reasons why I'm considering selling: 1.) Pay off $70k of student loans between my wife and I which cost us $1,200 per month in minimum payments and 2.) Gain $150k of capital in preparation for a market that I expect to be...choppy...and 3.) $600k for this property is about $50-$75k more than what others have sold for in the area and I don't expect that to be available again for another 18-36 months.
Also, I live in NJ - I may be wrong, but from what I understand, capital gains is exempt for the first $250k every two years. If that's true, I shouldn't have to take a tax hit on the sale.
So you see what I mean:
Option 1: Keep a great, cash-flowing, appreciating asset
Option 2: Sell while the market is high, pay off all debts, and prepare capital for similar / alternative investments.