you haven't said what year but its usually the driveshaft cv joints.mine was the inner one and you had to replace the whole driveshaft as joint is part of the shaft.mines a mk2
If you need further advice try here
www.fgoc.co.uk
this is the ford galaxy owners club and we have meets etc.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.