The 3.9 Liter is a 5.2L minus 2 cylinders. The firing order is usually stamped into the intake. Looking at the front of the truck, the left side should be evens 2, 4, 6 and odds on the right side 1, 3, 5. If it is fuel injected it is posted on the injector plugs. That helps with correct location of the cylinders. It is sometimes on the distributor cap. Trace the plug wires back to the distributor cap and fix the cross. Little tip on changing wires and distributor cap. Take cap off with wires connected then replace 1 at a time.