"Most" of the time, the aftermarket keys are worthless. All they do is to re-index the torsion bar to allow you to put a greater pre-load on it, thus raising the front end. It's really not recommended to go past 1-2 inches with just a torsion bar crank because of the associated driveline and steering angles you'll be dealing with. Most of the Rangers out there have enough adjustment left in the stock keys to get that 1-2" so that you don't need aftermarket keys. There is also the possibility that if your stock keys are maxed out, someone may have already cranked them. You'd have to compare your current ride height against spec to know for sure.
To answer your question, there is a special tool to hold the key from snapping back at you when you remove the bolt. What you really need is a method of "relaxing" the torsion bar (and key) in a controlled manner. I have heard of people using a jack under the key as well, but I don't think I would do that myself. Have you checked any of the parts stores that loan/rent tools. You might be able to borrow or rent one so you don't have to buy.
Good Luck
-krug
