TBH given your description, all the above answers are equally relevant, because you need to supply more info. You say you front 'diff has gone', it can fail in many ways as it is not just simple diff. The 'diff' is contained within the IRD that Nodge refers to. As Nodge says the diff component is very very robust and very rarely fails.... the other parts though fail regularly. The IRD has outputs to both front wheels and a rear pinion that drives the prop shaft the the rear axle - this is a frequent failure point - when it goes, the diff component is still intact so if you try to drive the car it will move but usually makes horrendous noises - even if you try to push the car. The driver's side output is a pretty simple drive, but the passenger side uses a splined shaft. If the splines 'go', you might not even notice because the noise may settle down and although that side of the diff may free wheel, there is a direct drive still to the rear axle.
So given these symptoms, there's no need to replace the gearbox - unless its the splines on there to the IRD that have gone or the fault is actually in the gearbox not IRD, and if you replace the gearbox to fix the IRD, you may as well try fitting one from a G Wagon!
For decent advice therefore, you need to say what the symptoms are. Why you believe its 'gone' and what noise and drive behaviour you're seeing, etc.
If it is the IRD, its also something else that's caused it. If you post what symptoms you've got, you'll get some decent advice on getting back on the road.