I have a 31 spline FRPP trac-lok. Only $189 from www.gefracing.com .Some people live by these, some don't. The thing thats good about these is there strong, cheap, and if you trash it at the strip. Its only $59 in parts to fix it. Its quiet as the factory 8.8. it held up to my friends 02 gt launching at 6000 rpms, on et streets, launching on the spray.
A trac-lok, or a locker? Your thread title says one thing, and the post says another.
Locker Pros: Best possible straight-line performance
Locker Cons: Worst possible everything-else performance
The biggest cost is some streetability. It will hook great at the drags, but you aren't going to be able to accelerate through turns any more or take them at significant speed without risk to yourself and others. Punching the gas halfway through a left turn is just going to give you snap oversteer with the axles locked. With the axles unlocked its like having a V6 rearend in the car, and you will just incinerate the inside tire. Sounds like fun, not.
That having been said, a good friend of mine runs a Powertrax diff and seems to like it. It takes some getting used to to train your driving habits to it but he is pretty pleased. I believe it uses wheel speed instead of air to lock the axles.