2000 Yukon TCC Not Locking

I have a 2000 Yukon SLE with towing package, that the lock-up torque converter doesn't seem to be engaging. At highway 50 mph the 5.3 litre rpm is 1700. I will maintain speed and depress the brake pedal, the rpms will remain constant. I have also engaged the Tow/Haul button at highway speed and the rpms remain the same? This is a 4X4 with 132,000 miles, a 5.3 litre and 4 speed auto.

Welcome to AF, only a full scan with a trans function scanner can determine if the tcc is being commanded and and is in fact working, if not this will almost always set a code.