i replaced my alternator shortly after i bought my 99 xj sport in March 2011. it went bad again in Feb 2012. luckily it was under warranty. i know it's a good brand that they are using and i had the pros put it in too. i just went out again today so i know something else is going on. any ideas?

 

Is it failing mechanically (bearings, etc), or electronically?

FWIW it isn't unusual for "remanufactured" alternators to fail. Some brands more than others.
Often they only replace bearings, brushes and shine up the slip rings. Sometimes just the front bearing is replaced. Chinese bearings don't last long. The orig. diodes and regulator are often left as-is to fail down the road.