Truck and trailer near max load, still need more gear...

Some would disagree, but if all else is equal to a 1 ton truck (minus suspension and tires) you would be fine going over on your truck's 9100lb GVWR within reason, as long you do NOT go over your rear axle and tire ratings (you would hit your max tire load way before reaching the axle rating I would guess).

This has been talked about in great length many times, and it all comes out that the only numbers that really safely matter if all else is equal, is the actual/real axle and tire load limits.