Before you go re-packing your bearings or spending lots of money on
new ones...
Take your van on a nice long stretch of SMOOTH road where you can hear
well.
get it going in third or so with low engine speed to maximize your
ability to hear the noise.  Put your ear up to the speedometer,
or as close as you can get without gettin your head stuck in the
steering wheel.
If you think the sound might be coming from the speedo,
(I thought mine was wheel noise for a while too) try driving with the 
speedo cable disconnected, does it go away?
If it's in the speedo, you may be ably to get rid of it just by
cleaning the thing out with an old toothbrush.

Cheers

<sstones@io.org>
SStones             Toronto,  Ontario.