McMurray defeated Ashton Lewis Jr. by 0.229 seconds in a crash-filled event at this 1.366-mile track. Mike Bliss was third, followed by Truex and Matt Kenseth.
Last Saturday , McMurray gave team owner Chip Ganassi his first Busch Series with a win at Phoenix International Raceway,. In this one, McMurray gave Rusty Wallace his first series triumph as a team owner.
McMurray, who is also dating Wallace's daughter, will split the driving duties next year with Jeremy Mayfield on Wallace's team.
McMurray's Dodge Intrepid was the class of the field late, but he had to hold off a fierce battle with Lewis's Chevrolet Monte Carlo.
Truex, who was on the pole after Friday's qualifying was rained out, has built an insurmountable 257-point lead over 19-year-old rookie Kyle Busch with one race remaining.
After entering with a 161-point advantage, Truex was assured the title when Busch was involved in two incidents, including a crash that knocked him out after 110 laps, and finished 33rd.
There were eight caution periods for 35 laps in the opener of a doubleheader here as Friday night's NASCAR Craftsman Series race was rained out and rescheduled for Saturday evening. McMurray, who averaged 109.711 miles per hour, led twice for 72 laps.
Aston Lewis' second-place finish was his best of the season as he desperately tried to win before his family-owned team closes its doors at the end of the season. Lewis will move to the Team Rensi car next year.