Did you install a used tranny w/ the LSD already in it? What car did the tranny come out of? Do you have ABS? These two answers are needed to know where your problem might lie.
I have the same issue. The p5 diff had a speedo gear on it. The msp diff didn't. I tried placing it on the msp diff and it didn't quite fit right. So I left it off knowing the speedo wouldn't work. When I get a chance I'm gonna do some research to see if the speedo sensor is diff on the msp.
So the msp uses the abs sensor for speed. Given I don't have abs I'm gonna need to remove the diff and install that speedo gear to get my speedo back. Ill do without for the time being lol