i can tell u that i was quoted about $700 to rebuild a diff and put the lsd in, however that was supplying my own lsd center. hope that gives u a bit of an idea
not 100% sure but i think your diff gears the problem unless you take off the cover n have a look at the gear, cause bearing noise do this all the time where as gear noise started when you reach 60 n above.
i went to sa race and sam hooked me up with a fully customised lsd diff new axles, retensioned it to 180ftlbs(standard=85ftlbs)but it was costly at around $1700 but it works so well no effort required.