The problem may be the Cam Synchronizer (shaft bearings) going bad. When I was in AZ a couple of years ago, I was experiencing an occational miss/harsh tranny jerk. I replaced the cam synchronizer with a new Cardone unit from Autozone. The bearing assembly was worn and very stiff to turn by hand. That resolved my problem. I had about 126,000 miles on the van at that time. No issues since and I'm at about 169,000 miles and just returned from another trip to AZ a couple of weeks ago.

 

How difficult is that to replace? I have "average" mechanic skills, but I've never delved into an engine.

 

It's actually not that difficult. It took me about 2 hours to do mine start to finish. The new cardone unit (part #84-S2601) was $10 more than a rebuilt unit and it includes the cam position sensor.

1. Drain the cooling system down
2. Remove serpentine belt out of the way.
3. Unbolt the P/S pump and move over out of the way.
4. Remove coolant tube that Ford put just in the way of removing the cam synchronizer.
5. Remove the cam position sensor.
mark the cam synchronizer housing and the engine block with your favorite color sharpie.
6. Put a mark on the new cam synchronizer in the same place as the old one.
7. Put the new unit in so that the mark on the new unit lines up with the mark on the block (no need for the sensor install tool with the marks).
8. Tighten the new unit down and install the new sensor.
9. You may need to put a new O-Ring gasket on the coolant tube, I just used some copper-plus silicone and reused the old o-ring.
10. Put everything back together and top your cooling system off.

Take your time, you should be fine.