Yesterday I had a catastrophic failure in my drivetrain which blew a hole in the top of the Swingarm casing and caused the entire final drive and wheel to twist a good 10-15° clockwise. The strut was presumably the only thing that stopped the whole assembly from folding over completely. I was going ~85 mph, when I began to notice a quickly worsening vibration, and found my rear end engulfed in smoke when I pulled over. Pictures are certain to follow.

Sounds nasty and frightening!
But oh what a story it will make someday.

Paralever?  U-joint failure?

Those swingarms are pretty rugged, but have their limits to what they can take.   I got rear-ended by a car last fall, broke the final drive off the Paralever swingarm.  Like you, the shock was the only thing that kept the rear wheel with the bike, not that I needed it while tumbling down the median.

Glad you're OK.

Year and model?

I know of several paralever U-joints 'splodin but I've never heard of a swing arm failing.

Can't wait to see the pics.

87 K100 RS

Holy cow! Never seen anything like that aside from Ks that have been impacted by cars and it's understandable why a swing arm might fail due to impact but to have one just fail of its own accord is a whole different story.

What's all of that black stuff on the brake reservoir? Bits of tire?

Yes, you can see where the sidewall was melting against the shock. I managed to get it in the shop and am working on disassembling it all now. I'm suspecting the driveshaft failed due to unlubed splines on the transmission side. Needless to say I am fervently searching for a suitable parts bike!

I think it's more likely that the U-joint blew up.  Which is pretty rare on the 2V Ks. The U-joint is sealed so there's no real preventative maintenance that can prevent that. But when it's removed for other maintenance it's a good idea to rotate it on both axes to see if any lumpiness is present.

If splines fail they just strip and spin.  And the rear drive shaft splines are where spline wear usually occurs, not the front. This of course assumes that whoever was in there last put things back together properly.  The front drive shaft splines have a clip in them and if the last person in there didn't get the front of the shaft all of the way on then I have no idea what havoc that might wreak.

I have periodically lubed the rear splines but have never opened it up any further. I've owned the bike since '09

I'm with Duck on this more than likely "U" joint. If the front splines had stripped noise and lack of drive would be the result.
It is not a big job to lube the front shaft splines and check the "U" joint while you are doing the rear spline lube. 
Regards Martin.

Seen a few do that but never damage the swing arm. When a U joint lets go it is usually explosive.

Different manufacturer, but I've seen a Guzzi cardan joint let go so badly that it distorted a 4mm thick steel swingarm housing and removed a large chunk from the back of the gearbox. I was riding it at the time... The gearbox and swingarm were pressed back into service with dents and bits missing for another 100,000+ miles...  

On another occasion I was asked to strip-service an Moto Guzzi 1000SP of unknown provenance. I took it out for a good thrash to see what might be wrong with it before the stripdown. On stripdown I discovered I'd been doing 120mph on a cardan joint with a complete set of bearings missing from one side...

So yes, having seen the destruction that an exploding cardan joint can do to a steel housing, I'm not surprised that the UJ almost managed to exit via the top of the swingarm casting.

Same manufacturer different model. Coming home from the Pigs and Pines run a few weeks ago on my R100R, drive totally disappeared, suspecting the clutch had failed. On removing the driveshaft (which was in 2 pieces) I suspect the front UJ had collapsed causing the Driveshaft to fail due to excessive vibrations. Luckily the UJ hung in there and did not let go. In this case I suspect the UJ. Cheers Tom

I'm very grateful for all the tips, and now i have renewed hope I can salvage at least the gearbox from this mess. The final drive had been dripping a little oil for some time now so I'll probably try to swap that out with another unit.

I'd guess that you're correct that the transmission probably survived.  If the output shaft spins normally and the seal on it isn't leaking then you're probably good to go.

Please post pictures of the internal carnage when you get the swing arm off.

What caused the wheel to lean over and rub on the swingarm? Did the swingarm casting break away from the final drive?

Negative, the casing twisted once the structural integrity was compromised. This was probably due more to the heat buildup from u-joint fragments riding between the driveshaft and casing than than from the hole itself. Speed was definitely a major factor.

I am not at all surprised at that swing arm twisting, it sure took a beating.
I have seen one twisted  in a crash that wasn't all that bad, bike came down sideways and twisted it about 3° but the wheel was left straight.

When I turned the Swingarm over *doh* I realized the whole underside had split open as well. There is only about an inch of unbroken, but visibly stressed metal holding it together on either side. Could have been a lot worse