BUCHVILLEMAN -
Unless you're planning on doing a lot of hard core wheeling (rock, water fall, etc.) and major mod to your vehicle, a front locker is probably a luxury, not a necessity. If you're going to do moderate to difficult wheeling, a rear locker is more than adequate for most situations.
I'm sure you'll get lots of opinions regarding rear lockers and they will vary. For my pesonal use, which is moderate wheeling, I choose the Powertrax No-Slip because of the low cost, ease of operation on pavement, it does not have the clutch plates to wear out in a few years, and it is a locker. Others who are harder on their rig have choosen the Detroil Locker, which is bullet proof from all the comments I have heard, it costs a little more than the No-Slip. There are also those that choose the ARB locker because of the versatility of being able to turn it on/off but is is the most expensive option. With the ARB option, the air compressor can also be used as a pump for your tires with the right accessories. I believe there are two other selectable lockers on the market now: 1) OX locker and 2) an electric locker by TracTec.
The No-Slip is an automatic full-time locker. It's a new design so it's more giving on pavement, almost transparent in operation. There are no clutch plates in the No-Slip so there is nothing to wear out.
The Gear-Less Locker from TracTec is also an automatic full-time locker. It has clutch plates so it's more forgiving on pavement than the No-Slip but the draw back is the plates could wear out making the locker not as effective.
The spool is an entirely different animal. There is absolutely no give whatsoever. Both tires are turning at the exact same rate. It's probably not the best choice if you're using your vehicle as a daily driver.
If you decide to get one for the front also, make sure you ask for a locker to fit in a Dana 35 with reverse cut because it goes in the front.
A number of people on this board are running with the No-Slip (in the rear), do a search for "No-Slip", there was a survey a few weeks back and see if it's something you may be interested in.