Get the temp sending unit from the dealer. I have read a few articles where the people buying them from Autozone, Napa, crown, etc don't get accurate readings. Aftermarket ones are ~10$, but dealer one is ~25$ (the back of the head one). If buy the one that's in the thermostat housing it's closer to ~75$ and that's for the computer, not your gauge.
I replaced mine a month or two ago, along with some prestone superflush and never gets past that 210 mark, and I was just out 4wheeling saturday in Wickenburg, AZ (a little nw of Phoenix, but still hot).