I've look into this quite a bit, and here's what I've found.
The premier offroad GPS are by Lowrance. These are more geared toward offroad racing though, and they are large an bulky. I can't imagine how one would do a clean install in the JK.
Lowrance also makes a crossover GPS, called the XOG. It's similar to the Magellan crossover, but has better topo maps. search for magellan crossover lowrance xog and you'll come up with some good side by side reviews.
As NYJK posted, the best topo GPS available is the Garmin Oregon 400t. It uses topo maps to cast shadows on hill sides. This produces topo maps that are by far the easiest to see. At $600 I'm waiting till the competition puts something out. I figure in about 6 months, the price will be half of what it is. I'm hoping that Magellan integrates this technology into the Crossover.
Lowrance rep was at the ORW opening. I spent about an hour with him they do have a smaller version now. They have built in data and do street and waterways as well as take chips. I thought these were the bomb. Good luck with your decision. Here is the link. BTW around $299 or less......... This is on my short list.
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