mechttu,
try a couple of tests.
with the wipers off and in the parked position see if there is any play in the wipers (can you make them sweep back and forth).
next try this in the full up position (when they are closest to the side edges of the windshield). great thing about these old trucks is that you can get the wipers started and then just turn the ignition off where you want them to stop.
and you may try this in some intermediate positions as well.
You may have something stripped out, or you may have something loose.
I think your problem is either in the linkage or splines of item # 11 of the attached link.
http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=97or the splines in your wiper arms themselves may be worn out
I really don't think the spring in the wiper arm has anything to do with this.
