Currie's folks say a well-prepped trac-loc can stand up to quite a bit of abuse. Stock ones have a weak point in the cast cover over the clutch pack. So everyone (Currie, Randy's Ring and Pinion, etc.) makes billet covers. Currie said, "When you put that
kind of power to a stock trac lock the cover flexes a lot causing it to
crack around the ring gear bolts, and causing the clutch discs to only be
grabbing on the edges because the rest of the clutch contact surface is
flexing out of the way so it doesn't even get used. You also have to realize
that the trac lock will work for this application (my fastback), however the more
horsepower and torque you put to it, and the more you improve your traction,
the shorter the lifespan of the clutch pack gets." So, short answer is the modified trac-loc doesn't suck, but the DL is more burly.
I installed a powertrax locker this year. The $400 one that is smooth like a limited slip. I ride on curved roads in North Georgia and I love it. Newer technology than a Detroit Locker and well put together. I don't know if the Detroit Locker is stronger or not?
As far a traction and abuse goes, my locker has been great. Driving it on the street with soft side walls does make for a squimy ride tho. It would probly be fine with a set of radials instead of bia plys.
i've got a 9 inch and a detroit locker in my 68. All the components are from currie. I love the locker. It's not as smooth as a limited slip. Every now and then it will pop and sway. I drive in the rain all the time, Sometimes there isn't enough traction to cause the locker to differentiate so the inside tire will skip a little (like a spool), but as long as you don't gun it there isn't any problems. But i absolutely LOVE IT!!!! Another thing is that i have 4.11 gears that make driving in the rain very easy. They are real soft on the drivetrain.
I put in a powertrax no-slip in my 70 Mach1. It works great, unlock when you turn and locks up quick when you are going straight. In the rain its better than my open diff was, but if you give it some gas on the corner be ready to for some fun driving...