The closer you get to the max capacity of your tow vehicle, the more uncomfortable it gets to tow. The suspension will bounce and the back end of the suburban will sag. It takes longer to stop and you'll feel the wind and get pushed around by semis.
It's hard to upgrade the tow capacity on your vehicle because there are a lot of factors. You can replace the rear-end gears to get a better mechanical advantage, but you'll still have braking limitations and the frame and suspension will be stressed.
The only thing I can recommend is to take your rig to a scale and get an accurate weight of the vehicle and trailer. If you're close to the max rating, drive a little slower and give yourself more room to stop.