3.55 or 3.73 Gears?

Hey it's been a while since I've been on here.. got a question. My car developed a whining noise in the rear, shop thinks it might be a bearing. And I said well as long as you're back there, now's my chance to install traction-lock and maybe bump the gears! So should I go to 3.55 or 3.73? Bear in mind about 75% of my driving is on the highway and if I suddenly find myself going 65 mph at 3000 rpm's and getting 20 mpg, well I'm gonna be mad. Has anyone actually changed their gears, and how's the highway fuel economy now?
At least the traction lock will mean no more one wheel squeals.

At 65 mph I'm about 1700-1800 rpm's. I did some research and it looks like a stock auto 98 comes with 2.73 gears. The shop has not finished their diagnosis yet, they are not 100% sure what the problem is. I may just go ahead and say **** it, put in an 8.8 rear end with traction lock and steeper gears. Take care of it all at once. But like I said before, I want to retain decent fuel economy, otherwise what's the point of having a V6?

Update: I wussed out. I was talking with the guy at the shop, decided since I drive 50 miles per day on the highway, to go with 3.27 gears. Since I had 2.73's in there stock, this will be a mild (very mild) increase. I'm keeping the 7.5 rear end, because switching to 8.8 would have cost too much. I am getting the traction lock, so I'll have power to both rear wheels. And I'll have new bearings.
So this increase certainly won't stand my car up on its rear wheels.. I may not feel any difference at all. But I don't care. Basically I need this car to last me 2 or 3 more years until I'm done with college, then I'll get one of the 2011+ 5.0 Mustangs with 400hp, and that'll be my beast. Heck, even the V6 Mustang for 2011 will have over 300hp, isn't that cool??