It's been concensus-ed that my '89 740GL wagon has a slipping differential, a terrible thing as of last night.
Loud clanks and jerks coming from the area. Wouldn't start to do it until there was weight enough in the car but then would do it with as little load as we could manage. Seemed to be 'activated' by a sharp turn this morning, but was happy to keep on in straightaways, turns, and anywhere in between once it got going; the turn thing may be a red herring, as we were testing for just the left front end at the time and things rather 'developed'. There's no minimum speed for this problem, and it's so bad as to make me very uncomfortable even trying to move it a mile down the road to a friendly driveway as slow as I can go, by which I mean it's no idle tapping but a rough jerking and banging. Without opening anything up, there's nothing visibly wrong underneath.

So, he's a car person but without much experience in suspensions and drivetrains; I haven't much car experience, but spent a year working as a bowling alley mechanic and he's worked there for a good three or more, on terribly pre-plastic machines. We've worked out that it sounds and behaves as though the teeth of the differential gears are slipping.

Our local yard has only one wagon (does it have to be from a wagon?) and it's not a copy of mine. So I have two questions for the board:
Does this sound like a reasonable diagnosis, or is there something we really need to consider? I've told you what I can think of, but I'm sure a few pointed questions can get more from me.
The question that got me to learn just how to use Brickboard finally:
What vehicles can I pull the back end from that'll be compatible with mine, in the 'day's work' sense, rather than an extensive alteration route?

Thanks a ton.