If I remember correctly, SRAM bought Grip-Shift in the late '90's, and still use the same mechanicals. I haven't worked on anything newer than about 2001, but at that point they were still the same internals as they'd been since their original introduction.
I cannot think of any reason they'd need to be replaced in pairs.
It is possible you'd have to replace both the right shifter and the rear derailleur, since they're calibrated as a pair and SRAM might have changed the winding ratio, but even then, you could find an old rear shifter and it should work fine with your existing derailleur.