There is a TSB regarding multiple cylinder misfires due to sticking valves. The valves can stick due to carbon buildup on the valve stems. Can not attach anything now, but have a look at the TSB's at WJJeeps.com - Jeep Grand Cherokee WJ 1999-2004
They use some mopar cleaner, but seafoam or alike could help.
If that doesn't help, i would swap the injectors from cylinders 1 and 6 with an other position and see if the misfire follows. Worst case could be the PCM.