Tow weights are something that seems to cause a lot of arguments. U-haul seems especially conservative, but keep in mind they are generally the go-to option for people who have never towed a vehicle or even driven anything larger than a toyota camry, so from an insurance perspective they need to be cautious.
There are several sites that will offer suggestions to safe towing limits based on your vehicle. A google search for "Tow Ratings" will point you in that direction. Remember, that if you are exceeding what is considered a safe Combined weight and you get in an accident you will be held accountable. In fact your insurance will be considered invalid and it will all be out of your pocket. Especially bad if someone is injured or killed.
So according to one reference: (I am using a 99 classic because the chart doesn't show 96, but I figure fairly safe to believe they are similar)
I am assuming your truck is 2wd. If you have 4.10 gears you should be able to safely tow 8000lbs and if you have 3.73 gears you are limited to 6500lbs. A fair estimate for the T100 is 4000lbs. I believe U-hauls trailer weighs 2200lbs. This gives you 300lbs of safety on your load if you have the higher gear ratio. I consider that a tight margin if you were to drive through the mountains or any slopes... It is also assuming you have no extra weight in the vehicle (tools, people, coolers etc)
The 2500 suburban is more likely to have the 454 and that will have a rating of 8500lbs with the higher gears and 10,000lbs with the 4.10 gears...
For true legality I believe you need to look at the actual Gross Combined Weight Rating of the vehicle and then subtract the actual vehicle weight and the weight of the loaded trailer and see if you end up with a positive number...
I haven't looked really hard, but I haven't found a source that had an accurate GCWR. Don't mistake it for GVWR...