I had this problem on my GC too. At first it was the tailight, but then it never went away.
Found out that the wiring was bad for one of the taillights. Whenever I'd hit a bump it might jiggle the wire into working again, and then move again and go out a few minutes later.
[URL=http://staff.filesnetwork.com/madman/jeep.jpg]RIP The Big Red Jeep. 1994-2005. 201,993 Miles.[/URL]
[URL=http://staff.filesnetwork.com/madman/CIMG1846.JPG]Illuminating the Night[/URL]
As far as I know, I'm the only person to own a flame red liberty with the light bar, and until someone disproves that to me, I'm going to enjoy it.