The difference between your modified setup, and my stock setup, is primarily 1st gear. My 6-speed has a much lower (numerically higher) 1st gear ratio to get me started with 5200 lbs behind me. On gen 3 trucks, the tow package's 3.73 axle ratios are intended to do the same thing...help get you going from a stop. Once up to speed, I have no doubt that you cruise just fine. But the potential problem is overstressing the drivetrain when starting from a standstill. Pulling 5000 with only 3.55 gearing thru the 5-speed is more stress than than the design or development engineers intended. I would recommend that when you start from a stop, you go easy on the gas, and not try to accelerate with traffic. Just be gentle on the drivetrain as you get yourself moving. High throttle at low rpm is not good for your setup.