If you leave the company that has the 401(k) from which you obtained the loan, the loan would be satisfied with an offset distribution, not a deemed distribution, and would not constitute defaulting on the loan.
You would have to find out from the plan administrator what portion would be an offset distribution from the traditional account and what portion would be an offset distribution from the Roth account. You would need to take the answer to that into account to determine how much you would be wrecking your retirement savings and how much would be subject to tax and early-distribution penalty. The only portion of a distribution from the Roth account that would be taxable and subject to an early-distribution penalty is the portion that is earnings. A distribution from a Roth 401(k) is part basis and part earnings in the same proportions as the amounts in the entire Roth 401(k).
You have until the due date, including extensions, of your tax return for the year in which the offset distribution occurs to roll the money over to another retirement account, should you be able to come up with the money.
