speedometer problems

the speedo cable is FUBAR'd.
Remove the sender from the side of the tranny and try turning it by hand. you'll probably feel some resistance at some point, that's what it's catching on causing it to bounce.
My father's old 79 B300 used to bounce between 35 and 45, then clear up from 50 on up.
edit: unfortunaltely I think the speedo cable is a dealer item.

speed o

I had a problem with that and what it was,,was there is a wire that runs along the frame to the rear differential,,i was plowing one night and went over a pile of snow and broke the connection with that wire along with the abs wire,,so my abs/brake light came on,,i would release tilt all the was up,,then quickly move it back down,,it seemed to reset the situation,,i did splice in a new wire but it does it from time to time when raining,,try that though with the steering wheel tilt and see if that helps,,,

perhaps a van is different, but with 2wd 1500s, the speedo is fed by the gear driven cable, which is located on the driver's side of the tranny.