I would call this a toss up. I have 3.55 and do almost all highway, mainly longer road trips. They are nice when there is no significant headwind, as it just cruises no problem and gets good mileage too- I used to be consistently at 23 mpg. However, the problems come when you hook up anything in back. half of my road trips I'm towing a boat and im pretty loaded down for all of them. It does fine except when towing a boat although the boats I tow are not big boats- a 16' skiff, which did fine on a flat road, but killed in the hills, and the 16' Hobie it really has trouble with considering that it dosn't weigh much more than 1k lbs, although the pontoons do stick out so the boat does not draft off the truck, which kills when going against wind. So my point is that with the 3.55 (and maybe all explorers, I'm not sure) you will feel anything you tow, and some if it will make a big difference, but once you get up to speed on a flat road w/o wind (which never happens for me) its allright. But even if you never tow and do highway mostly with just you in the truck, lets say it costs minimum of $500 to do switch pumpkins. the pay off for that would be very long because you may get what, like 1-2 mpg at most due to the higher gearing? not to mention the questionable quality of junckyard parts, as this part is pretty important. so IMO its not really worth it. And also, the 3.55 is sooooo slow when you accelerate from a stop, its really annoying. I would stick with the 3.73, and if you have a limited slip, definitly stick with that as the 3.55 open is not a good choice for a 4x4 that ever goes off the road.
