Z2FAST8

I have a 96 6sp w/ 4.10 gears that I bought a few months back. It has always whined at highway speeds especially at light throttle cruising but no whining when foot is off accelerator. Well when I got the oil changed I went ahead and had them change rear fluid which they used 80/90 and limited slip addititive (is this the correct fluid?). A day or so later upon 3/4 throttle a bang happened and the car wouldn't move in 3rd gear. Pulled over and could get it to move in 1st but would make a clunk in the rear and it does this when pushing it. And the ABS in OP light comes on as well.

I'm not very knowledgable when it comes to rear ends so any light you could shed on this would be appreciated. Thanks