Factory limited slips (can't comment on aftermarket ones) wouldn't be totally engaging at highway speeds(low engine rpms). I've had them in all my trucks and suvs over the yrs,except for the '96 Cherokee I now own,and some do engage at lower rpms than others but still not at the low rpms of on road travel. I do miss having one in my Cherokee and am seriously considering some type of rear traction control device.
Here's a good read(takes a few seconds to download):
http://www.offroaders.com/tech/limited-slip-lockers-differentials.htm
