I am looking for a GPS unit that does the "usual stuff," with a display and ability to "talk," but with an added feature - the ability for me to load a predefined route (probably a shapefile created by ESRI's ARCMap software) into the unit, then hand it to someone and tell them to drive where the GPS tells you to go.
Anyone know of a GPS unit with those capabilities?
Rather than a shapefile, you probably want to look for one that supports KML, which is a similar format that is supported by Google Earth, etc and thus are more likely to be found than one that supports shapefiles. I am not sure if there are any GPSes that do support such things, however, as I'm sure it would be difficult for the software to parse a shapefile/KML into something easily translatable into turn by turn directions.
You can upload a custom route to most Garmin Nuvi devices. Some of the newer ones even let you create the custom route on the device itself.
https://support.garmin.com/support/searchSupport/case.faces?caseId={27edcdb0-0c86-11dd-dc9c-000000000000}