Broken grip shifter?

I have a 2000 Specialized Hardrock with grip shifters. Yesterday when I was riding, I noticed that it no longer clicks into place. I can roll it fowards and backwards, but it slowly irgrates back to the highest gear. I have to clamp down on it and squeeze it harder than I would like to to keep it in the prefered gear and even then it will slip into other gears. This isn't a good thing for my hands (risking numbness, cramps).

I think I broke my shifter. Should I attempt to install a new one myself? I live with someone who's mechanically inclined (though slow as molasses when it comes to projects). Or should I take it in to the shop? Have I even diagnosed the problem correctly?

Thanks!!!