At about 70,000 miles I replaced my factory plugs. I used Bosch Platinum and really didn't notice any change good or bad.
Recently (last 3-4 weeks) I have really started to notice rough idling, hesitation and an occasional stumble. Tonight, at 94,000 miles, I replaced the plug wires and plugs. I went with Motorcraft replacement parts. Plugs were AGSF 34EE, gapped to .053.
These replacements resulted in a BIG improvement in driveability and performance.
I do suspect it was the wires more so than the plugs. But, based on my experience, I suggest using the Motorcraft plugs - and wires too for that matter.
New plugs and wires cost about $80.00. Not enough money to be saved by choosing another brand to warranty the uncertainty, at least in my book.
So, I'm still happy as a lark with my truck.
2000 model at 94,000 miles and NOTHING has broken - only routine consumable items replaced.
I hope this info is helpful to others.

 

Tommorrow I will be installing my new plugs and wires.I also went with Motorcraft parts from my local Ford dealer.
Motorcraft wires-F6PZ*12259*BC-$55.01
Motorcraft plugs-AWSF*32C*F4-$1.98 ea. gapped at .045"

Total with tax-$71.59

 

The list is long of people who have encountered driveability problems using Bosch plugs and/or wires in their Fords. I hang out a lot in the Ranger & 3.0L Forums and see post after post describing people's negative experiences using Bosch ignition parts in their 3.0L/4.0L engines.

 

Just like BrianA spoke of my truck was getting some hesitation and a shudder. I thought the shudder was in the transmission because it only did it in over drive and going up slight grades. The check engine light came on and Autozone checked the code and it said "Catalyst Effeciency below Threshold". They said my cats were bad. I almost replaced them but thought I'd let the Ford garage put it on their computer. They said the #3 plug and wire was bad. I was using Split-Fire plugs and Beldon wires I had got thru my Napa store. I had about 33,000 on the wires and plugs. I replaced them and solved the shutter and hesitation. I'm thankful that I did not replace the tranny or complete exhaust system. I still would have had the problem. My truck now has 188,000 on it and running great. Next the brakes.