You may be at the limit or exceeding the weight/cargo capacity of your Suburban. I tried looking up the specs for the TR-26FQB but was unable to find them on current Hearland site; so I'm using the next closest thing: a TR-26RLSS. According to Trailer Life Towing Guide you have a 6500 lbs max towing capacity, GVWR for a TR-26RLSS is 7600 lbs. Now I know that most people are not going to loading the trailer to the max but with a dry weight of 6151 lbs; do you think you'll exceed your 349 difference? I do.
Look at the Mfg info plate on the drivers door frame and find the GVWR for the Suburban and the actual weight if one is listed. Also look for GCWR, that should be there as well. The Suburban and the trailer can not weight more than the GCWR number.