Got the truck back today and it was in fact the throttle position sensor that went on the fritz, repaired under warranty.
Also had them change all the fluids while it was in (truck is 4 yrs old w/ 80k kms): synthetic oil & filter, Trans fluid & filter, transfer case fluid, brake fluid, & coolant. Rear diff fluid was changed last year, and leaking front diff seal was fixed under warranty a couple years ago with fluid topped up then.
Cost $850 for all the fluids changed - I was expecting more. Could have saved $$$ by doing it myself, but the truck doesn't fit in the garage and I'm not keen to lie on my back in our gravel driveway. I plan on keepng the truck forever, so don't mind spending some money on preventive maintenance.
And great service from our local dealer, as usual: the service writer actually tried to talk me out of changing the brake fluid, saying GM doesn't recommend it (lifetime fill
![Roll Eyes ::)](https://www.autos.ca/forum/Smileys/CarTalk/rolleyes.gif)
). I think it's smart anyway, and was pleasantly surprised to find the brake pedal much firmer now. Guess it needed to be bled anyway.