2006 Yukon XL, 3/4 ton with towing package, will not idle above 1200 rpm/10-15 mph

check ICS for correct operation
engine idle speed is 850 -950 rpms for auto and 750-850 for manual
at 1200 rpms you are not on the idle circuits but on power circuits
run the fault codes

If front park lights are working highly unlikely headlight switch is the issue. Is the vehicle equipped with a trailer towing package and wiring to the rear? I would start there. This vehicle may also have a taillight module located under the vehicle on the passenger rear attached in the wheel well area that goes bad

camshaft sensor located above crankshaft pulley, the camshaft sensor mounted on the front timing cover above the crankshaft pulley.

Make sure that the ****** isn't slipping.

Check fuel pressure regulator vacuum line for fuel in it. Replace FP regulator if it does.

clen the idle air control vale and passage with air intake cleaner

Unless you have the 6.0 ltr motor and towing package the maximum should be about 7,500 lbs. This will be less if you have the small V8 or 6cyl motor It all depends on the motor and towing package. The 6.0ltr will give you above 10,000lbs towing capability, if your truck is set up for this. I don't think you can get the 6.0 in a half ton model though, it must be 3/4 to one ton models.

2006 Sienna Standard towing specs:

2WD or 4WD: Max Tongue Weight: 200 lbs
Max trailer and cargo weight: 2,000 lbs;
(over 1,000 lbs requires trailer brakes)

Max trailer and cargo weight: 3,500 lbs WITH the factory tow package

Thanks for using FixYa!

If the scan will narrow down which o2 sensor it is and replace it. If it has more than one on that year that is. Under load you will probable have a richer fuel mix. If the sensor is going going bad it may only get out of spec reading under heavy load and warmer temps. I have had similar issues with GM and it was easily fixed.

check coolant level! look for coolant loss