You are after a lower ratio (higher number) diff, not a higher ratio diff as you have put it. Yes, I know that's confusing.
I agree with WBT56, 3.9's are too low for street use, leave them from dragstrip specials.
Single spinner or open diffs will transmit equal torque to both axles, so if one side losses traction it will not be able to resist the torque and spin until traction is regained.
LSD diffs work the same until one axle tries to rotate noticably faster than the other. This differential in rotational speed is limited by clutch packs or viscous fluid ensuring that some rotational forces are sent to both axles at all times.
Spools are permanently locked diffs. That is there is no differential action and both wheels are forced to rotate at the same speed all the time. No good if you need to turn corners and you respect your tyres, axles, diffs etc. Not for street use.
Locking diffs (Detroit locker etc). Will allow differential action for turning by disengaging the outside wheel and allowing it to turn faster (meaning all the torque then goes through the inside wheel). Under power the slower turning wheel will be receiving all the torque or the diff will lock turning both wheels at the same speed. Can cause trouble on high powered road cars with engaging/disengaging of rear drive wheels - spooky on fast sweepers at full throttle. I wouldn't recommend this for a powerful street car either, fine for four wheel drives that require the locking action off-road at slow speeds and that acknowledge the limitation on road.
Territory - WHEELS COTY