The tow package's main parts are the hitch, wiring harness, and coolers for engine and transmission oils. The axle ratio is a seperate item. I also order the tow package to keep the warranty in effect if the tranny burns out; I know a few guys who ran into the service guys who knew the manual said the tow ratings were only if "properly equiped."
I've got a 5.4 with a 3.55LS and pull a 3000 lb bass boat. I can tell in the hilly areas that the top gear is too low, and must come out of overdrive. I'll go back to the 3.73 next time. My previous truck had a 3.73 and it would not come out of overdrive as much. It would also hold speed on the freeways better, with less shifting, and got better gas mileage.
So, if you only pull the trailer occassionally, the 3.55 will be OK; if you pull often, and deal with hills or high altitudes, get the 3.73.