It is unclear to me at this point exactly where the closure on the south end of I-15 is at, but before I-15 was constructed, traffic went on US-91. That road still exists (although it is now a county road in poor shape, and not appropriate for large trucks).
It runs from Beaver Dam, Arizona to St. George looping west of I-15.
http://en.wikipedia.org/wiki/U.S._Route_91#Arizona
Perhaps it will help.