2006 GMC Yukon, 3/4 ton with tow package will not idle above 1200 rpms/10-15 mph

There is no connection between the idle revs and revs at speed .The revs for 60 mph sound about right but the idle revs are controlled by the idle speed sensor and various other sensors Have the fault codes read Replace the faulty units with
genuine one because most aftermarket units are not set for OEM cpu units and are not completely compatible.

Get the gforce chip out of there & see what happens

You put the problem in there with the mentioned items

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

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

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Clean the trottle body & Check spark plugs.

Your catylatic converter may be clogged if your loosing power or top speed is less than it was. You can check this when its running with a battery operated infared heat gauge. the temperature of the muffler should be around the same temp. as the catylatic converter.

your vehicle will need a complete computer diagnostics to determine this problem and its solution. there are a lot of unknown variables to be able to accurately begin to solve this.

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.

It sounds like your low on fluid, or your transmission filter is dirty and the front pump is not getting enough oil supply. Worst is your front pump is going bad. 
Check the fluid level and see if it's burnt. Good luck