I want to increase the top speed of the truck (I know I'm sacrificing torque but that will be fixed later on). Where would I buy a taller gear for my diff, or what is all the work I would need to do? My truck is a 1996 3.9 A/T rwd SLT.

 

What differential ya have in it? Is that the 8.25? What gears are in it now? What size tires?

 

I have 8.25 with a 3.55 ratio with 16 inch tires, I'm thinking about going down to a 3.07.

 

16 inch wheels, I should hope your tires are bigger than that.....

Just going to larger tires would effectively do the same thing, just not quite to the degree that changing to a much lower gear would. And keep in mind, changing gears is not an inexpensive proposition. If you haven't done it before, or, have someone that can help you that has, doing it yourself is not without peril. Do it wrong, and you get to do it all over again. And, unfortunately, it is way to easy to do it wrong...... Paying someone else to do it though, you are lookin' at 1200-1600 bucks to do it right.