...,would doing the front or rear be better for offroading? Or do you have to install front and rear at the same time?
If you're not planning to re-gear any time soon, take your pick. "Locker in the front will be less noticeable ON-Road(If you've never driven a locker equipped truck on road, there are some quirks
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But do some research here. What gears do you have, and what rear axle do you have? Are you planning to re-gear any time soon? The carrier break for the front axle is 3.55/3.73. The carrier break for the rear depends on the axle. If you want to re-gear later, and have a a Cry 8.25, do the rear first, as the locker you get will work for any gears you buy.(carrier break for a 8.25 is around 2.54; any gear in a jeep is already lower then that, so it doesn't matter)
If you're planning on re-gearing soon, save up and do it all the same time. Doing a locker install (labor) is about 1/2 the price of a full gear setup. It's bad enough to pay for the locker install, then turn around and pay to re-setup for gears, you may end up buying a new locker because the new gears need a new carrier.
Also, if i dont intend to do any rock crawling, is it still a good thing to spend money on?
A locker(front or rear) will make a serious improvement off-road. I locked my CJ-5(rear) and almost quit using 4WD. If you don't do a lot of off-road, and don't go in the rocks, you might consider a Detroit Tru-Trac(gear driven limited slip)
Most people here will say get the locker, but the L/S preforms extremely well as long as you keep both tires on the ground.