Hi. If all four (or in your case 2) wheels are locked up (braking) and you go down hill with the diff locked then if the surface is slippery and you take your foot off the brake the wheels might not speed up to the road speed and you'd be skidding and not rolling down the hill. This is more applicable to snow and ice really, but mud is similar and I think soft sand would be the same.
Anyway, yes get a decent Volvo and you're done!
(then fit the fiddler brakes!!!)

Cheers,
Matt

I've used my diff lock here (Egypt), in sand, and I do notice it. I admit that ice and snow are something I've not experienced particularly (too bloody cold). On mud and loose surfaces the wheels do slip round, but again, it is still harder to turn, proportional to the grip available, though not like on tarmac.

When I do use it it's normally in a fairly straight line as when I first started with it I noticed when I wanted to turn the wheels had a tendency to dig in more. The rears scooping out the sand more and the fronts resisting the turn more and ploughing the sand up on the edges on the outside of the turn as the rears are trying to drive the car straight.

So guess that nearly settles it then, 940 Volvo with LSD, lifted suspension(+1 to 2") so all that's left is to chose between petrol or diesel. Not looking at going too extreme from the beaten track(and will carry 40 litre spare), so would I get away with petrol option(with catalyst removed) or should I spend a bit more and find a diesel?

If you can get a Detroit Tru-Trac LSD for the volvo then it is superb - no clutches, only gears and very very reliable. I had one on the front of my Defender (with a Detroit no-spin automatic locker on the rear). Both worked superbly and I never noticed any noises or problems in use. I suspect you can get one as Eaton seem to list most vehicles in their applications.
Gil