Victim of city driving - no high or low ratio

After sorting out the faulty starter motor the old beast of a td 2.5 90 (1987 model) has done it again. A case of cityitus syndrome?

I get the revs, can change gear but, get no motion. I had it (very helpfully) looked at by a kind gent who suggested that there was a diff lock issue - not going into high or low ratio.

So, is this easy to fix? And, is it expensive to repair (ie, should I cancel christmas?). Any suggestions?

Many thanks also to the gent (in the series 3) who towed me and the kids home - I owe you for that kindness.

remove rear cover 6 bolts and check input gear splines,diff lock has nothing to do with hi /low apart from they share same lever ,try in difff lock as broken half shaft or cv can leave you standing ,axle with spinning prop has problem if checked when in gear with out diff engaged

Jamesmartin - Thanks for the suggestions which I'll check out over the next couple of days.