My recollection is that my diff-lock-off experience is like your
diff-lock-on experience.  I have not had to engage the diff lock at all
since I bought the van last spring, tested it, and at that time the diff
lock worked in sync w/ the dash light; "on" means engaged.  Perhaps I'm on
the verge of some drive train failure. 

On Wed, 3 Jan 1996, Tom Forhan wrote:
> 
> Diff lock off: No shuddering or binding. Steering effort is high 
> toward the stops, which I always thought was due to the expanded 
> footprint of the oversize tires. Other than that, nice smooth circles.
> 
> Diff lock on: Shudders, enough that I did not want to keep on doing 
> this to the vehicle. Shuddering definately decreases as I approach 
> straight line. No visible tire scuffs  on the asphalt, but it is
> not a dry surface.
>