I was curious of the length because I am interested in purchasing a TT that same size, maybe a bit smaller, but from the sounds of it, it seems like your truck was struggling a bit. My truck isnt lifted nor do i have large tires on it and mine is also 4x4 (if that even makes a difference). I am also in Wisconsin where I don't have to worry about mountains. Just getting an idea of how mine might tow without lift/tires/gears

 

The truck struggled before I re-geared the rear differential to help with the 35s. Had I had a stock diameter tire, I think it would have been fine with the existing 3.77 rear end. I feel pretty good about what I'm pulling with the truck now.

 

4x4's will have a larger GVWR rating then a 4x2. I've seen this in all manufacturers trucks.
As discussed, I think the issue here is with the larger 35's that the OP has; this is effecting the towing. Even with the rear gear change, it takes a lot on the stock drive train to move them; adding a TT is compounding it even more.