The fluctuating idle is definitely caused by too high of an idle but if your truck is running bad it will idle fine one day then high enough the next to cause the fluctuations.

If you want to eliminate the problem completely there is a lead you can cut that sends a brake signal to the ECM but I'm not entirely sure though that I would cut it if I had an auto transmission as that is the situation it is designed for supposedly. And this is a band aid for another problem at best.

However I dealt with the idle problem for 12 years and was more than happy to cut the wire. It fixed the problem completely.