I have recently encountered a problem like this.  It has only happened
on long highway trips, over an hour.  When I pull off the highway into
say a gas station and turn sharply on pavement, it takes a lot of
throttle to make the vanagon move.  It feels the same as if I was
driving with the diff lock on.  It has only happend a few times.  I
have  tried engaging the diff lock and then disengaging again and it
went away.  Once I didn't do anything and it went away.

Can the diff lock engage by itself?  Heat? Vacuum leak?  If the diff
lock is engaged without using the vacuum actuator, would the light on
the dash come on?  Mine doesn't, only when I pull the knob.

I can feel the one of the rear tires dragging, so I am fairly sure it
isn't the VC.

Peter Pistner
91 Synchro