I replaced my driveshaft at 145K miles. The carrier bearings were not in good shape, but the U-joints weren't too bad. The reason for replacement was a mod thing. I went with a 2-pc aluminum shaft which was lighter, and it eliminated one of the U-joints and carrier bearings. At that time (almost 6 years ago) it cost $770 including some machine shop work to adapt the rear diff flange, and I had to send them my old shaft.
http://www.dynotechengineering.com/