It's a lot easier to diagnose with a scan tool. Everything is basically tied into the PCM, BCM, etc via some sort of databus, CAN, LAN, etc on newer vehicles, which is how the different components communicate.
These asre the related parts for the cruise control system:
The powertrain control module (PCM)
The On/Off switch
The Resume/Accel switch
The Set/Coast switch
The torque converter clutch (TCC)/brake switch
The stop lamp switch
The throttle actuator control (TAC) module
The vehicle speed sensor
There are only a few basic things you can check for w/o a scan tool.
1. Check to make sure your brakelamps are functioning 100% properly. The cruise control system takes into consideration the state of the brakelight switch, whether or not it is opened or closed in order to send the correct signal to the PCM.
2. Remove the fuse and re-insert. Sometimes a non-conductive film will build-up between the contacts and cause intermittent problems from low voltage.
A lot of times the on/off switch or resume/accel portion goes bad before anything.
There is also a TSB (Bulletin No.: 03-06-04-012A) concerning the integrity of connectors which 06 and prior light duty trucks are included. Among the related symptoms are intermittent/inoperative cruise control.