Mechanical limited slip differentials works a bit differently than BLD. The BLD system brakes the wheel that is spinning to send the power to the opposite wheel that has traction. This means it stops or slows down a wheel to send power to the other wheel. This is good for slow off roading or when one tire is in the air. Although this works a lot better than an open diff, slowing down a wheel to gain traction is not ideal in sand, gravel, or other kind of surfaces that you need momentum to get you through. Another downside is the uneven brake wear if constantly used and you can overheat your brakes if it is overused, but this is extremely rare.
With the Torsen gear style LSD found in our Ram 2500/3500s, it does not brake the wheel to send power to the other. What it does is multiplies the torque traction of the spinning wheel to send more turning force to the wheel with traction depending on the the torque bias of the unit. IIRC, I think the torque bias ratio of the Trac-Rite GT in our trucks is 3:1 which means it can send up to three times the torque to the wheel with traction without slowing down a wheel. It also responds very quickly, and starts to transfer torque before BLD can activate. This limited slip is ideal for sand, gravel or other slippery surfaces where differentiation of torque bias is needed without hindering any forward momentum. The downside to this style of LSD is when one wheel is in the air. You cannot multiply torque if there is none to multiply.
This is where the BLD kicks in and aids the Torsen style LSD. With a tire in the air the Torsen LSD is helpless, but the BLD kicks in to apply braking force it give the LSD the torque it needs to multiply and send to the other wheel. Back in the day, guys with LSD used to use their park brake in these scenarios to brake the wheel enough to send power to the other wheel which worked fairly well depending on how good you were at it. This was called the "poor mans locker". Now BLD does it for you and even better since it only applies braking force to one wheel. Seperately these two systems are good for the certain situations they are best at, but together they will make your truck damn near unstoppable since they compensate each others downfalls. They don't replace lockers, but together they are damn close to being just as capable.
Hers is a quick video on the kind of LSD we have in our trucks. Although it is the Eaton True-Trac, the AAM Trac-Rite is the same and acts the same way.
http://youtu.be/lZmsY2YvVsc
