I would totally agree with Terry on this one.  I go to the local U-Pull-it
all the time.  All you have to do is check the pulleys on the alternators
there and almost every single one is bad.  In my experience Bosch Alternators
(on Vanagons) last about 100k miles before the back bearing (or bushing) goes
bad.  Most folks just think their belts aren't tight, but it is usually the
alternator (or the waterpump, or both!).
The next time you have your belts loose on the alternator and waterpump try
this:
Put one finger at 12 oclock and the other at 6 oclock and try to rock the
pulley up and down.  Hear/feel that clunk?  Time for a rebuilt alternator (or
to start saving for one).  You can do the same test on your waterpump (it
shouldn't clunk, but if the pulley moves then it is time to replace).  Also
good to try this test with your fingers at 3 and 9 oclock while rocking the
pulley from side to side.
You can only do this test when the belts are loose or off.  When they are
tight everything will seem honky dory.  Also if the alternator or waterpump
makes a chirping noise, then this is also a tell tale sign of toastage.

I have rebuilt, Bosch Alternators for $150 with a $75 core charge.  Comes
with a 1 year warranty and 2 years roadside assitance.  These work right the
first time, everytime.  I don't sell cheaper rebuilds as they aren't reliably
good.

I also have new, German, waterpumps that come with the gaskets and hoses that
you need to install them.  List special on these is $50.

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242