Yes, you can replace one lifter. It may or may not solve the problem, depending on the condition of the camshaft lobe it's riding on. You'll need to do a "break-in" run of 15-20 minutes at a fast idle of 2,000 TPM or so before idling the engine immediately after startup, the same way as is done with a new cam and lifter installation. This allows the new lifter to establish a running fit with the cam lobe.
Just loosen the rocker arm nut, remove the pushrod, swap lifters, and re-adjust the preload on the lifter. Then button everything up so the break-in run can be done before starting the engine.
Jerry
