rite the diff locked up today but it freed up just as quick its been goin slowly for the pased year now and again if i pull off fast and the car wheel spins it then crawls n picks up again...but its got the mot in 4 days time i was just wondering if this could cause a fail its all that is apearing to be wrong and i got a spare gearbox in the garage but dont realy want the trouble to fit it incase thers more wrong that i cant see...

AFAIK this isn't one of the things checked as part of the MOT, the MOT checker may put this in an advisory notice. They may not even notice if they crawl it in and out of the test bay.