Coming with the trailer tow package I suspect you have the 3:55 rear end which would be good news...
I had a 2006 SCab 5.4L with the 3:55 rear axle and it had a rated towing capacity of 8,600#. The problem was, as noted above, payload or the lack thereof. It pulled my 5,600# 22' TT like a dream but was right at max payload doing so despite being 3,000# under the towing capacity and a couple thousand under the GCWR it was maxed out.
I used this combo with nothing but good results for 4.5 years all over the west but was always conscious of weight distribution when loading {not over loading}. You should be able to run the Vin on the internet and verify your actual differential ratio. If it is 3:31 your towing capability will be negatively impacted.
Good luck.
:C