I have a 02 Xterra with a 5 speed. it gets about 14 mpg on the highway and the motor is screemin on the highway. are there any after market or stock overdrive set ups for me???

It has an overdrive, 5th gear is .811:1 Much taller and there won't be enough power to use in the first place.

You could get larger tires, that would lower your overall ratio, but your indicated speed-to-RPM correlation will not change. Indicated 70 will still turn 3300 RPM (or whatnot) but you'll actually be doing 75 or so.