Downhill control

[Mosey]

I have 2002 Yukon 5.3L automatic 4x4. Whenever I go anywhere, the first half mile is all downhill (I live in the mountains in Colorado). I have to ride the brakes the whole way down even with the trans in low gear because the idle speed is too high. If I go back up, then go down again after it's warmed up, I can put the trans in low gear and idle down just fine. It idles fast when it's cold, then slows down after it's warmed up. Is there a way to adjust the idle speed? Also, is there any way to force the torque converter to lock in? Either one or both would help. I can put it in low range and it idles down nice and slow, but sometimes I don't want to have to use 4WD and it doesn't have a 2WD low range. I also have a 1999 Chevy pickup 5.3L automatic 4x4. When it's cold, it idles slower and holds the truck back better than the Yukon and I don't have to ride the brakes to keep it to the speed I want. I don't understand why the 2 are so different. Thanks in advance for any help.