Your TIC Agreement should spell out what happens in the case of a default on the part of a co-tenant, but what is written in the agreement and what actually happens can be very different. The process of pursuing legal action to get to a "forced-sale" situation can be time-consuming and costly, and you have the additional complication of the group loan when it comes to selling if you get that far.
Another component to your situation is whether or not the other unit can be sold for more than is owed, and what are the ramifications of selling with regards to the group loan.
I would consult with a Real Estate attorney - we can recommend a couple if you like - and then consult with a konwledgeable and experiences Real Estate Broker who knows about these kinds of properties and issues and make your decisions from there. Although being honest I'm not super-excited about getting into another messy situation, I have personal experience with a similar situation and can give you advice from the marketing perspective. We are currently checking out a new bank that has much better rates than currently available and that could help a lot.
Lance King/Owner-Managing Broker