Engine details please? 4.0l or 4.7l?
A misfire is usuallly caused on the HT side of the ignition and causes the cylinder to fire out of sequence therefore things that can cause it are:
Faulty HT Leads
Weak coil - Reduced spark at the plug
Worn/fouled plugs - i.e oiled up!
Broken electrodes off the plug
Dampnes in the HT lead - remove them and bake in the oven then spray liberally with Duck Oil or WD40 whilst hot! (It won't go up in flames! Done this many times.)
Dampness in the Dizzy (Distributor Cap) again bake in oven..
Hairline cracks in the Dizzy..
Faulty electrode pack...
Hope this gives you some ideas where to look.