Seedo jumps when range seletor is switched up and down while parked

I have never seen that on my W900 nor any W900 I drove.
Many times I sit in the truck I have activated the range selector especially when I was troubleshooting (like you)
and that doesn't happen. It didn't matter whether the clutch was in or not or sitting still with the engine off but the ignition ON. Can't duplicate your issue...

It's interesting and off hand the only thing I can thing of is that the speed sensor is on the back of the trans....
but how or why?

 

Never had that happen but probably depends what kind of tranny and controls you have.......most likely a line or fitting on the tranny is leaking air or it could be at the button in the shifter knob too......fix that first and see if that cures things, if not pull out the speed sensor and clean it off.....it should be somewhere in the tailstock of the tranny.......might be best to service the speed sensor anyways......they are cheap to replace!! Also make sure none of the wires running from the speed sensor to the ecm haven't been rubbed bare and are grounding out......kinda sounds like 2 issues going on here!!

 

I'm guessing a bad/loose conection on speed sensor (probably ground side). when you move the range selector it changes the quality of ground. When I bought my 05 T-600, the dash lights would brighten and the static on my CB would change "tone" when I pushed the clutch far enough to engage the clutch brake, found a broken ground strap between frame and cab, now it has 2. The air leak in the shifter is a loose/cracked connection/line under the knob.

 

Check for a loose yoke nut on the back of the trans, that will allow the tone ring to spin.