diff problems
 

so i just bought an 85 rx7 the other day, the previous owner told me that it has the LSD in it (im not sure though). anyway i was getting on the freeway last night and started hearing a horrible whiny whirly noise coming from the rear end along with a teeth shattering vibration. before i could get to the next exit the bolts holding the driveshaft to the diff all sheared, and i was stuck. so we towed it home with a dolley (rear wheels on the ground) and it seemed to roll just fine. and had no problem pushing it up onto the dolley. anyone know what could have caused this?

Im kinda working on the conclusion that i need a new diff unless i hear otherwise, which brings me to my next question. i was under the impression that this year has drum brakes on the back but mine has discs. is that normal or is that something unique to the models with the LSD? if it is unique would i be able to swap in an open diff (pumpkin and all) and keep the discs? im just assuming it will be easier for me to find an open than an LSD, and i dont have the tools or confidence to replace just the diff (ie. adjusting backlash, rotational torque, etc) so i just want to find a complete ready to go rear end to drop in there.