defender issues

Hi all. New to the site, have had a 90 with 200tdi defender engine in for 8ish months now just for green laning basically have had few problems but been able to sort myself up to now, but how ever was out laning today on a regular lane I go on, got half way up and she really started to struggle getting anywhere on what she normally finds pretty easy. Then I realised that I only had power going to the rear axle so I reversed back down to the road to find that I only have drive in diff lock and no longer in high or low box, when high or low box are selected it does slightly sound as if its trying to engage but nothing happens. any ideas? 200TDI with LT77 box I think.