Our van has the 5.7L (and I think different transmission) and 4.10 rear; our experience won’t be the same as with your van. Our trailer runs 6-7k loaded (5400 unloaded) and our cargo van weighs less and has less added payload than a typical passenger van with passengers.
It tows well on the flat, we struggle to stay at the posted speed limit on the uphills of I81 through PA. I have found tires make a difference in stability while towing; I have found the Michelins we’ve had provided the best stability.
Note: Extended GM vans have longer wheelbases than regular length vans (the rear overhang is the same) unlike Ford E vans.