I say replace both...When my '01 coupe had about 3000 miles on it, I ran over some debris, and cut one of the rear tires. I replaced only one, with a similar GY run-flat. Problem was, the OE GYs wore at a quicker rate that the replacement tire, and in time, my posi unit wasn't happy with the difference in circumferences of the 2 tires.
It's the the driver side rear tire. The resident corvette expert at the dealership told me unless I have money to burn there is no reason to replace both rears.