I would ask who is installing the belts on this beast.  Could be a case of
"over-tighten-its" or they could be using the Armstrong method of vanagon
service (if it ain't broke, tighten it until it breaks).
I would also replace the pulley on the water pump (or at least inspect it).
Could possibly be bent or somehow out of balance.
Does anyone have any idea that maybe a resistance in the cooling system (like
a restricted hose or partially clogged radiator) could put some extra strain
on the pump?  Just an idea.

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