Here is a video of the belt slipping. You can barely hear it, but you can definitely see the belt vibrating when it is slipping.
http://s84.photobucket.com/albums/k13/rollguy/?action=view&current=IMG_1112.mp4

Any ideas?

All Diesel Fleet:
1985 Mitsubishi 2.3 Turbodiesel Pick-up, 1983 GMC Van- 6.2 Diesel.
Benzs: 1995 E300 diesel, 1980 300SD, 1982 300CD, 1983 300D, 1985 190D.
1980 AMC Eagle Limited Wagon powered by: 3.6 Litre I6 VM Turbo Diesel Engine (one of fewer than 10 made, only 2 known to still exist)