If it is the diff, (and I have my doubts) then when it goes you will experience a big bang some grinding and clanking then loss of drive.
Not dangerous in itself, but if your pulling onto a motorway or similar it could be very hairy.
Just drive it and save up for a second hand box from the scrappy, £700 sounds a lot but if it is genuine recon box with 12 months ticket, new clutch kit, slave etc etc it soon adds up for a Joe public buyer.
I would get the CV's done, you cant check them without taking them off and worse case if its the diff then you know the cv's are all good
