Yes, you must have them rebuilt in pairs.
As well as wear and tear the design of the shock internals has changed too.
The original Koni 82-1701 and 82-1601 have a fault which can make them sieze solid. When Koni rebuild them they replace the internal parts with parts of a newer design.
My front left shock was so stiff I could only just move the rod, the right side was fine.
I had both front shocks rebuilt - it didn't cost much.
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