Do a factory reset and if after the reset, the firmware needs re-updating, do that for starters.
If that doesn't work then I'd get it sent off for repair.
I suspect that there's either some kind of obstruction under the button, like a grain of sand or some dirt that's stopping it working correctly, or that the connection is worn from over use. How many clicks does is have?
For all my repairs I've used
Fixation - very prompt service, usually under a week turnaround. You basically fill out a
printable repair form and send that with your packaged item, making note of serial numbers etc so everything tallies up. You can instruct them to go ahead regardless of cost (they then contact you for payment) or you can tick the box on the form saying you want a quote over the phone and then work from there.
I've had a few repairs done; new mirror box on a D2x, new rubber grip, new hotshoe (and grip). All have been done to the highest standards and you'd have never been known the camera had been repaired.
Be mindful that basically any repair on the D2x/xs bodies where there needs to be internal access (I suspect this is one of those) requires the rubber grips and sealing to be removed so that will have to be replaced. Think it works out about £100 to do that before the actual repair of the others parts; crazy I know, but that's the price you pay for having an all-weather, pro body
