rear diff
 

i have an 01 sierra 4x4. the rear diff is a posy but it really isn't. the rear diff in gmc's works by when one tire starts to spin in engages the opposite side and they both start spinning. i've herd these diffs can break when u giver and the tires spin and the opposite tire trys to engage. how strong are these diff can they actually snap ?

Those are limited slip, very popular in these trucks and Camaro's and T/A's. They can with stand alot of torque so I wouldn't worry too much unless your goin above 500hp.

It's all done by computer, at least in my 05 it is. It won't engage when the tire is spinning too fast.