fixed alty,now 4x4!MU 3.1td

well got the mu through the test, fixed the alty which cured almost all electrical faults.
now lwft with 2, the first is the ever up/down speedo and weirdly after fixing the alty i noticed this. At start up 4x4 green light on as usual,goes off after small rev of engine. Select 4x4 and no light and 4x4 def isnt coming in! I thought they were mechinical rather than electrical. The mu dosent have locking hubs but like the mk2 or 3 trooper can change whilst stationary. I just cant figure this out as the 4x4 was working great prior to the alty change. My local garage did my cv boots recently if in anyway this may have nudged some ac/dc connector loose from underneath.
its a great little motor and though the lsd is nice i could do with the 4x4 back for the fields.
Any help would be appreciated.
Thanks to all that reply.
Regards
Cameron

Does it have a shift on the fly button on the gear lever? if it does it will be a vac related problem and that could be a hose from the vac pump at the rear of the alternator.