sorry man.
really, don't mean to troll your threads, it's just a thougth...
if i were in your situation, i'd start thinking about a solution like this: ...
...i'd try to find at the junkyard a complete electronic speedo, with all its needed ancilliaries; like dash, sensor, elctronic management unit (???if there's any).
donno what car from.
then i'd made a toothed ring, (sorta abs like), to be set on the solid member of the half-shaft, just near the diff.
this could be either laser-cut (in here is very cheap), or made out of an aluminium ring (less weight), with tapped holes in it, where put some bolts or grain or something ironish, just to excite the standard speed sensor.
little interference. or play with grains/bolts to fix 'em in position, (depending on what solution you have in mind).
same number of teeth of toothed wheel of the car you take the rig from.
diy a bracket to hold the sensor in the correct position.
if you want to laser-cut it and don't have any cad available, just pm me the dimensions you need. for me take out the unquoted .dwg needed, is a 10min job... (and pleased to help if i can).
however i've NEVER PERSONALLY MESSED WITH ELECTRONIC SPEEDOS, this would be a new situation for me too. so donno sh*t about the real feasibility of my bullsh*ts above.
donno if it's cheaper too...
just trying to help with another input.
good luck with it. keep us posted and let me know if i can help...