You don't need to replace the tensioner. The bearing is $10 from NAPA and the whole job should take 30 min max. I attached a link so you can see pictures
If for some reason the link dosn't work, it's posted on Yahoo groups Tdi list under tensioner bearing in the picture section.
First take the belt off and remove the pulley. You don't need to remove the whole tensioner. You will see a small snap ring on the back side. It may have holes so you can use a snap ring pliers. It not, you need to pick it out. It's a bit of a pain but not bad. Once out, use a 1" scoket to drift the bearing out. Once out use the old bearing to drift the new one in. The bearing is NAPA # 6203-2RSJ and is the same for D90, V8 & Tdi as well as Disco 1. Cheers