Got mine from Braydons Volvo in London. There's a box on ebay at the moment, but its £300.
You need to look at the whole train starting from the gearbox output, unbolt the angle gear and check the splines,then check the angle gear for play, then check the propshaft cv joints and centre bearing. Not sure how to check the viscous coupling, but I've not yet seen anyone with one that has gone, and the rear diff is off a 960, so there shouldn't be anything wrong with that unless the oil has drained out of it.
My problem was with the output shaft from the gearbox. The splines on it had stripped and also on the connecting sleeve. My angle gear is spot on with no play in it, so that wasn't the problem.
I sourced a second hand gearbox from a place in london, cost me £200, and I bought a new connecting sleeve for £80 from Rufe. I spent 3 days taking the engine out and swapping the gearbox out, but that was because there were a few problems.
See my thread here
Good luck with it and give us a shout if you get stuck