You are well within the towing capacity of a Yukon. There are a lot of U-Haul dealers in San Antonio. Go to a different dealer and don't tell them what you are going to haul. And I don't know what direction you are going out of San Antonio, but you won't be getting into any mountains if you only go 150 miles. At one time I had a 5.3 GMC several years ago. I pulled a lot heavier loads on my own trailer with no problems.
