Wow, a 318 with 3.55's and your pulling a car hauler with a Jeep on it...about #6000. I'd say your doing real good if your even getting up to highway speeds. That combination is NOT the recipe for success.
The trailer tow package does not give you the ability to tow bigger trailers, it makes your towing connection bigger, stronger, safer. The ability to tow is a combination of power and gearing, and that is something your missing on both accounts.
If you want to pull that trailer, with that truck, skip the 3.92's and go straight to 4.10's. It will pull much better.....then you can look at other mods, like tuner, exhaust, intake, etc. but without the gearing, you'll be spending a bunch of cash, and just not getting big results.
If your pulling that much, and often, consider buying a truck that is better designed for the application....like a Hemi with 3.92 or 4.10 gears, or just about anything diesel. If your looking 2500 gas, they usually come with 3.73's, and that trailer load will be close to the comfortable limit with those gears, so even there, consider one with 4.10's.
Even with a standard trans, you won't be using the top gear....much.
BTW - I have an 08 Ram 2500 Hemi 3.73 6sp std. I regulariy pull a closed in 12' trailer weighing 6400lbs. I use top gear down hill/with wind. I also use 1st gear to start off (clutch saver). Pulls great, mileage sucks...wish I'd ordered it with 4.10's.
I've just replaced it with a 2010 1500 Hemi 3.92's....we'll see!