Does such an animal exist? I want to run a vdo electronic speedometer and I can't find a sending unit.

Along the lines of this ?

http://www.summitracing.com/parts/vdo-340011/overview/

That makes sense. Does it pick up of the stock pinion gear?

Looks like it screws in place of the speedometer cable.

Yep, it screws onto the factory speedo adapter instead of the cable.

I am working on a buddy's car right now actually with a VDO speedo and he uses one of those style sending units. (Not a Mopar, but same idea)

My 92 D250 has both mechanical and electric output for the speedo/odo.
Perhaps that could be of some use.

The pulse rate of the sender has to match the speedo being used.

autometer speedos are the same - drive 2 miles and hit the calibrate button. 518 trans I got out of a van had a electric sender (2 wire) that screwed into the stock cable housing.

Just use a gps speedo. Uses a small antenna.

My Autometer can be calibrated. You can basically follow another car and drive exactly 2 miles, or you figure out the amount of pulses for that distance, then turn the sending unit until that number pops up on the screen then save.

With that being said, my speedometer runs off of magnets on the driveshaft using a magnetic pickup for a rostra cruise control kit, and a VSS buffer module for an 80's 5.0 TBI firebird. Works amazing.

I run a great app on my iPhone called 'Speedometer'. Gives a dead on gps reading for speed and odometer distance,,,,among many other attributes.

Really handy to determine how far out of calibration you are when swapping tires, rear ends or even transmissions,,,,drive gears for speedometer cables often have differing tooth counts just as will driven gears.