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You'll DEFINITLEY want new gears for it. I had the 3.73's in mine and even with the V10 if felt under-geared. Don't get me wrong, it had plenty of power and never had trouble. But it would hunt gears while towing. If you are gonna get it, do yourself a favor and spend some extra bucks (look at around 1K) and get some 4.30's put in it. You'll be much happier with it.
I'll second BFR, I had a V10 with 3.73's towing a 7k lb 22 ft trailer and it does it, but nothing like the V10 with 4.30's that I have now. If you're planning on towing in the mountains or even tall hills you will be very disappointed.
My personal opinion is that the 5.4L isn't enough engine for the SuperDuty. It's the "base" engine to entice you to pony up the extra money for the V10 or PSD. The fuel milage is fractionally better than the V10 and the upgrade potential and re-sale value are limited. But that's my opinion