NEED HELP!!! JUST INSTALLED MY J30 DIFF

[240sx"95"]

I got a s14 that i just installed a j30 diff early today(5bolt). I put in the diff and the axelson. I went to give it a test drive and when i am in low gears and i brake the car starts to rattle/shake and it sounds like the diff is making very weird noise. i made sure that there is the right amount of fluid in there. i re tightened all the bolts and checked everything. i dont know if it could be this but when i was puttin in one of the axels 3 of the balls fell out, but i just put them right back in and it looked fine.. anyone has any ideas???