So - I figured this out.
When i got the Truck about 10 months ago it had a very loose throttle cable, so I tightened it. Apparently a bit too much.
The other (quite hot) day it started to race again- I popped the hood and pressed hard on the throttle (body) and it instantly dropped to "normal idle". So my throttle was opening, Loosened the cable a touch and here we are. No more runaway throttle.
No problem through winter, no problem in daily driving, but when it the truck got hot as described above, the throttle cable sheath must expand just enough to crack the throttle.
*feeling rather stupid*