Vic,
I do not have an answer, only some suggestions.  You probably know that any
odd sound in a vehicle is like a leak in a roof in that it is not always
originating from where it seems to be.  Since this clicking can be heard as
the van creeps forward, have someone move it so that you can jog along beside
it and make certain that the sound is coming from the rear.  It may be coming
from the front and could be a bad FRONT wheel bearing.

One time I had a 240 Volvo that made a buzzing sound at a certain RPM.  My
wife could drive the thing to Alaska and not even notice this buzzing but, it
would drive me nuts.  I was certain the sound was emanating from the rear; I
just couldn't isolate it.  My wife figured I had finally lost it when I asked
her to drive around with me in the trunk so I could once and for all silence
that damn buzzing.  When she realized that she could get even with me for
anything I had ever done to her by hitting every pothole she could find, she
willing slammed the trunk lid.  After that ride from hell, all I had to show
for my bright idea was a bunch of bruises.  Months later, I reached to crank
open the sunroof, just as that buzz started and it immediately stopped!  It
was coming from the knob on the sunroof crank.  I slid an O-ring over the
knob and thanked the Lord.
Rich