Ive had my fair few 4x4 issues and quite often, the commonest issues are the following>
Earth issue ( check all the earth connections including the ones on the fuel rail as a few times this has caused the light of death to come on) clean up all the earths
Check the power steering fluid level and if its low its poss you might have air in the system..
Check on cav16 web site under bleeding the hydraulic system> http://www.cav16.co.uk/transfer/
Should explain how to remove trapped air in the accumilator block as this can happen if the fluid level gets low and traps air..

2 good places to start
Another>>Also, if you have the early turbo its poss the transfer bx temp switch has failed! This happens if the fluid in the bx has over heated! There is a way of testing the switch but tbh, i cant remember right now

If its failed it will need replacing..