I have a new distributor on mine and new tps and I have the same problem. It just started a few weeks ago and only happens in 4th or 5th gear when it's in lower rpms under a load (I first noticed it going up a long incline in the mountains, as it started slowing down in 5th gear, it started shaking, almost like the clutch was slipping). When I would downshift to 4th, it was fine again until it started slowing and it did the same thing again.
When I bought the truck a year ago, it had a bad tps and did the same thing, but much worse. I have the 4.9L. Let me know if the tfi solves your problem