You must have the 4.8L?
Your hunch is correct that the conversion weight should be deducted from the towing capacity. The factory passenger version was 600 pounds heavier than the cargo version. A fancy conversion is probably more like 800 pounds vs a cargo van.
You are correct the Express is too wide to dolly. And too heavy. And wrong-wheel-drive. And too heavy to pull with a Trailblazer anyways.
The U-haul auto trailers weigh about 3,000 pounds, so you will be over your factory tow rating. But the good news is that the limit is powertrain-limited, not chassis limited (chassis maxes out at 10k). So there's nothing unsafe about pulling the TrailBlazer IF you have an aftermarket Class-5 hitch receiver. You'll just be slower on the hills, and you'll want to watch your temp gauge on long pulls. But do check your hitch receiver. The stock receiver is only rated for 5,000 pounds without weight distribution, which you can't use on U-haul trailer because of surge brakes.
