I have seen similar kits for mounting aftermarket cruise control, race car data logging, digital speedomoeters.... however, I always pictured them mounting the magnets to the driveshaft tube, not the yoke. The front (transmision) yoke seems like a pretty good idea, as long as you can center a 1.875" - to - 2" collar ring around it. That might require you to turn the yoke in a lathe for a tight fit.
http://www.powertraincontrolsolutions.com/catalog/product_info.php?products_id=101
Here is a slightly dfferent application, with some electronics theory, again sensing at the differential yoke:
http://www.beaststwo.org/truck/index.shtml