hj60 front diff not engaging - big snow tomorrow!!!

G'day,

Here in Belgium we're expecting the first serious snow of the winter in the next 24 hours, and I'm expecting the first serious row of the season with my dear wife if I can't get her to the shop for supplies, and more to the point I'm about to run out of beer!
I've got an '89 hj60 4.0 flat diesel which I bought from the UK on ebay a few months ago - I've not owned a 4x4 before, nor a Toyota, so I was staggered at the sheer pleasure derived from just turning the key and driving the 1000 k back home without incident. I stopped worrying about it after about 5 minutes - what a thing!!!!
Apart from locking my only keys inside as I boarded the ferry - one of the Dutch crew gave me an old wire coat hanger and said,'you have five minutes', so I said, 'what if I can't open it?', and from behind his back he produced a lump hammer!! - I've had no problems apart from this recent failure of the front diff to engage. The 4wd light does not show green when pushed in, although a 'click' behind the dashboard is discernable. At around the time the problem first occured the brake light started to flicker on and off which according to the handbook suggests a vacuum problem, although the fluid levels are OK and all the pedal pushing/holding down/ on/off tests indicate the brakes aren't an issue.
Is there any way I can make this front diff engage.
I'm down to half a case of German Beer, some frozen Chicken and minus 8 Degrees.
Any help or advice would be truly welcome.

Best wishes,

Paul