This unit is not on the market quite yet, but if you could get your hands on it, I would endorse it
http://www.nisuautomotive.com/index.html It is result of the recent commerialisation of the open source RoadRunner Car PC frontend by its creator Guino. See
http://www.mp3car.com/vbulletin/road-runner/I use RoadRunner as my Front end and it rocks. If you read up on it, one of their test rigs was a Jeep and extensive off road testing.
So if you can work out how to get a PC in your car, the Odyssey Navigator software is very easy to use and very reasonably priced. see
http://www.twig.com.au. All you need is a PC and a USB GPS receiver.
Here is my Car PC with a 7" LCD touch screen in a modified case stuffed in the dash showing the RoadRunner .
And here is a screen dump of Oddyssey Navigator (before I registered it). It runs happilly in the background while you change to other screens. When a voice announcement is made, the music player is muted so you can hear the directions.