Speedo Problems - Again!

[strat]

I imagine this one's been up before... Had the diff changed on my chim a couple of months ago and since i got it back the speedo doesn't work properly. It's fine up to about 60 or 70 then it just pings round up to about 180! I suppose there's a sensor problem or something... I would take it back to the garage but they are in bristol and the car is now in york. Doh!
ANy suggestions? Could it be an easy diy job or should i get it into a garage?
Cheers.