He is a salesman, thats all. You will have to do your own research. If you can get the VIN number you can have a GM dealer run it and get the answer on the axle ratio in the Burb. There should be a sticker on the drivers door that has the payload rating. You will have to take the weight of your family and any gear you plan to carry in the Burb and subtract that from the payload capacity, that is what you will have available for the tongue weight.
I don't think 120,000 miles is too bad. You didn't mention what engine is in it, 6.0 or 8.1.
About forgot to add that I would be surprised to see a 3.23 rear end in a 2500.