Thank you for your question. First check your plugs and wires on those cylinders. These engines are due for a tune up at 100,000 miles. With the age of the truck a tune up may be due. Doing all plug wires at once is usually most cost effective since once one goes the others usually are not far behind. Here is the firing order for you. Please reply with any questions you may still have.
Firing Order 1-3-7-2-6-5-4-8