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With a 5speed, by FAR the best bang for the buck is getting a friend to teach you to drive it correctly.
Except in 1st gear, regearing is EXACTLY the same as downshifting.
4bt all the way with a zf 5 speed. I seen a guy on www.pirate4x4.com do it to a 94 2wd ranger, but he switched it to dana 60s front and rear, so now its 4wd. His name was mooktank I beleive, he might even be on here. Said he was getting about 20 or so on the highway. I would throw one in mine but it wouldn't benefit me because I need wheel spin and not torque for my application.
I personally wouldn't drop money on gears. Unless your lifting it, bigger tires, etc etc.