Price of a 3.545 R200 differential?

Well things are starting to look like I need to replace my differential. Ever since I first drove the car I have had issues with intense driveline vibrations when cold and bad whining when decelerating in gear. I have recently developed a troubling situation where when I shift out of gear while in motion--especially in first and second gear-- for a solid second I hear and feel a loud knocking noise, which I now believe has something to do with my pinion gear. If you have had a similar situation and it turned out not to be the differential let me know as I would love to not blow through a diff for no reason, as vague as my symptoms may be. I am taking the car to a mechanic just to verify my guess.

My question is this: has anyone recently gone about purchasing, old or new, a differential with the same specs as the stock 84-86 turbo differential? LSDs cost way too much for me right now and I do not want to pull the trigger on the first ebay diff I find without understanding what the market value could be. Conversely, what could I expect from having someone rebuild my differential? I appreciate any response.