In late 70's when I first started towing. max trailer was usually recomended as being no more than 2x GRAWR.
With OP's truck if it is a 3900 rating. 8Klbs of max trailer, or the tail wags the dog.
For some of todays 35 series trucks, with 8500-9000 lb rear axels, upwards of 18K lbs of trailer.
In ALL cases, one needs to keep the RA with in spec. BUT, if at the max. this happened with a lot of folks back when the dmax came out, they bought 2500's with 15K tow ratings, loaded up the 5W, and found themselves with PLENTY of power, but were bottoming out the truck, due to too much wt on the RA.
No matter the gvwr of the truck, look at payload first, then tow rating.
Marty