Should be okay. Probably your weakest link is the brake system.
Official Ford spec GCWR for a 6.9 auto w/ 4.10 is 14,000lb (1985) which gives a max trailer weight of 7-8k depending on your truck light weight.
That being said, the 460 auto w/ 4.10 rating is 18,500lb, or ~10k trailer and that's all the same drivetrain aside from the engine which says to me that it's all factory rated from that kind of load and the 6.9 truck is derated due to lower power.
I pull 10k with some frequency and have gone as high as ~14k (trailer weight) with a 3.55 rear. Wouldn't really want to do that with a stock truck, but if you don't mind being slow the truck should handle it okay.
Just make sure the brakes are in good condition. I have hydroboost and an exhaust brake on my rig - would be much more hesitant to pull heavy without both.