Do I Need to Replace One of My Drivers?

I have a pair of PSB 5Ts, and I recently noticed that one of the 5-1/4" midrange drivers exhibits a slight, momentary, barely audible vibration in a certain frequency range. I picked-up on this recently while listening to Jack Johnson. I noticed it (and had to confirm by getting my ear a few inches from the speaker) during certain classical guitar passages in the lower ranges. Vocal and drum parts don't seem to cause it. I have switched my speakers from left to right, so I know it's with one speaker and not my amp. Any advice? If I need a new driver, what is the most cost-effective way to get one?
First pull out the driver and see if you can spot any cone damage. Cone repair may turn substantially cheaper than purchasing a new driver.
Have you checked if some part of the surround has come loose?

If it has it is easy enough to glue back down.
Also just try tightening the screws holding the driver in the speaker.
Often they come loose.
Now not TOO TIGHT, just tighten a little.
Usually the screws are plain wood threads in fiberboard, and trying to get too much tightness will only strip out the holes.

In fact check all your driver screws every year of so..
The do come loose.
Yes, check to see if there is any air escaping from behind the driver. I had a midrange vibrating once from air being pushed out. This driver had a seal that failed.