My late model 2008 Macbook Pro has been a gem, but all of a sudden the right fan started to give out, the left would be clocked at over 6000rpm while the right was dredging at barely 2000. It was time, after 4 years, to swap the components out.
It's an easy swap, took 10 minutes only because the screws are so dang small. Fans were in perfect condition, and my computer hasn't been this silent in years. I'd almost forgot what a new MBP is supposed to sound like - nothing! Silence again, my ears rejoice.
If you're going to replace one, replace both, it's a cheap swap and you'll save yourself in future woes. If one died, the other is right behind it. Remember to discharge any static electric build up with the metal plate apple installed.