You have a leak. Usual places are the gasket of the tank's sending unit (there was a recall on that one), the hoses (either the one from the pump to the rail or the return), the lines around the fuel filter (it's hidden behind a skid plate near the rear drivers' side wheel), or the fuel filler lines are cracked. Could also be the rollover valves acting up or the hoses from said valves (I just replaced mine today, as a matter of fact), but if you smell it WHILE you are driving the Jeep, my guess is that it is a rather LARGE leak, which is probably around the fuel pump of fuel filter...
My advise would be to drop the tank and check... It's really not that hard (took me about 2 hours to drop it, change the hoses, and put it back up all by myself today; but then again, this is the 5th time I drop it, so I pretty much know how to do it by now!). That way, you can check the hoses and all. You may want to start at the filter, though, as it is the simplest to check... Make sure the fuel pump is primed so that you can see (or smell) any leaks.