Detroit Truetrac
Overview: The Truetrac is a gear-driven limited slip that requires no special friction modifiers to operate. That means that the bozo at the Qwikee Lube can't destroy your spendy limited slip by putting the wrong oil in. Under power, torque is sent to both tires unless there's an excessive amount of resistance on one.
Street:
The Truetrac is very unobtrusive on the street. When gassed in a corner or turn, you may feel the slightest pull, but there's no jerking or violent engagement of any sort. When under heavy throttle in a turn, the inside tire will bark a little, but not under normal driving conditions. Turning radius and tight maneuvers are totally unaffected.
Off-road:
The Truetrac offers near locker-like traction in all but the gnarliest terrain. We've found that modulating the brakes a bit can help keep one tire from spinning when the suspension is crossed up. You can't drive as elegantly in the rocks as you can with a spool or locker, but you'll have to work pretty hard to get stuck. In loose dirt twisties where you're lifting a tire, a Truetrac will almost always get you through.
Good: Great traction on- and off-road and can use straight 90W gear oil. They're moderately priced and great for front and rear applications.
Bad: There are a lot of little parts inside, so it's not as strong as most full-case lockers. Also, it doesn't offer as much traction as a locker or spool, and you may slightly notice it operating during some street driving.
Power-Lok
Overview: The Power-Lok is a clutch-driven limited slip unit and is considered a stronger, heavy-duty version of the Trac-Lok. Power-Loks are commonly found as the optional limited slip in older Jeeps. New units are produced by Precision Gear.
Street:
When worn out, you'll think it's an open diff. But even new, it's hardly noticeable on the street.
Off-road:
Again, when the Power-Lok is worn out, you'll think it's an open diff. Modulating the brake pedal helps make the Power-Lok come alive and provide impressive slightly-better-than-open-carrier traction. We give them a solid "eh."
Good: Came factory in some M38A1s and other early Jeeps, so you may already have one. They're stronger than most other limited slip cases, and can be used in front and rear axles without adverse effects.
Bad: They require gear oil with a friction modifier and have wearable clutches, which can be replaced at home. Not for the extreme and not a replacement for a full locker.
Wrangler JK TFS Electric Locker
Overview: When Jeep upgraded the Wrangler line to the '07-up JK platform, the old pneumatically-actuated lockers of the TJ Rubicons got the heave-ho in favor of the new electronically-actuated selectable lockers. Although they only fit Rubicon Dana 44 housings, the TFS Dana 44 Rubicon lockers markedly simplify the installation of Rubicon axles in a non-stock application, requiring only a switched source of 12V power to actuate.
Street:
Hey, when it's off it's an open diff. No drawbacks here.
Off-road:
To be somewhat picky, we've found the JK Rubicon axles sometimes need to be locked 20-30 feet before you need them, rather than when you're already stuck. When locked, they effectively function as a spool, offering uncompromising traction in any and all terrain. That is, as long as your tires are up to the task.
Good: It's got millions of dollars in factory-funded research and development behind it. They just work.
Bad: They only work in JK Rubicon Dana 44 axle assemblies, so you can't just plunk one in your '76 Wagoneer front axle.