How canI force a co-owner of real property to sell?

You really have 2 issues here: (1) how to get you name off of a loan, and (2) how to force the sale of jointly owned property.

As to the first issue, you can ask your co-borrower to ask the lender to allow what is called a "novation" to remove your name; realistically however this is rarely allowed.  Still, it's worth a try.  A novation could be accomplished by your co-owner owner paying the mortgage for a period of time (typically 8-12 months), then if he/she is current and has never been late, and they can prove that hey were the only one paying the mortgage, your lender may allow them to remove your name from the mortgage without having to refinance.  Also, a novation may also be obtained if yor co-borrower "buys" your release by making a substantial payment to reduce the mortgage balance. As for the second issue,  you may have no choice but to pursue something in the law known as "partition". This is a legal remedy available to co-owners of property when they cannot agree as to ownership matters.  Partition allows for the division of property among co-owners; any co-owner can file for such an action.  Accordingly, if a property can be physically divided the Court will so instruct.  However, where division would be impracticable (as in the instance of a single family house) a Court would order a sale in lieu of partition and an equitable division of the proceeds among the co-owners.  Before doing so however, the court would permit one co-owner to purchase the interest of the remaining co-owner at fair market value.

Note: If the co-owner is a spouse, then that may change things.