The plot thickens!
Having stripped the entire front axle down and checking the prop, diff, half shafts, CVs we still havent found the issue. We did find that the clutch master cylinder was dry, oops. Wondering now whether the landy jumped out of gear and i couldnt then get the gearbox to engage into high or low properly unless i smacked it across to diff lock.
Alot of work for potentially a totally different fault.
On the plus side, the near side stub axle spline was wrecked and one of the swivel balls was leaking and the propshaft seal from the diff housing had a small leak too. All these will be sorteso not a total loss.