You would want to replace both rear calipers at the same time. The new one could have different characteristics than the old one, and next thing you know your 240 is wanting to jerk to the side for a moment while the other caliper catches up.
(the only time I ever replaced just one caliper, was on a '76 Malibu with 148k miles, and I didn't expect it to last much longer...plus I was young and dumb (now I am no longer that young)).

-Larry