Hmmm. . an interesting project. You may very well need the cruise servo motor in addition to the switch. And the wiring harness for the cruise may need to be connected to the ECM, since the wiring may not be there from the factory. But I believe it could be done, with careful research.
Best thing to do might be to just buy a throttle position sensor assembly from an SLT or Laramie, which (I think) includes the cruise servo. I'll have to check my service manual (don't have it with me at work!).