My first response would be: The Hungarian Revolution of 1848 succeeds, Austria loses its Hungarian territories, and then loses an Alt-Austro-Prussian war, and becomes forced to join Germany afterwards.
But, then I noticed the POD after 1850, and I realized that that wouldn't work. To be honest, it would be hard to get the other states to accept all of Austria-Hungary as part of Germany, although just the Austrian half (incl. Bohemia, etc.) could probably work, which is why I was thinking of a successful Hungarian revolution...
A Revolution is indeed the easiest solution, and 1848 came very close because the combination of the Italians and the Hungarians simultaneously fighting the Austrians was more than they could handle. It would be hard to inspire another uprising of that scale in the immediacy of the failure of 1848, as the retribution against the "48ers" was severe across Europe.
The simplest way after then, IMO, is to bring forward or push back an Austro-Prussian war to coincide with a general uprising of minority peoples in the Habsburg empire. Perhaps around the year 1860 or so....
If the Italians still go to war for the rest of Northern Italy, and the Hungarians are induced to revolt by the Prussians, then a decisive Prussian attack may push things beyond breaking point. At that point, the smartest (German) politicians in Vienna will start thinking about deals that result in submitting to Prussian hegemony in return for privileged positions in the new regime. A few well placed Prussian bribes amidst the chaos could see Austrian generals surrender their armies without a fight, and for statesmen to form a "capitulation faction" within the Habsburg Court, pressing for peace on Prussian terms. Ceasfire occurs when the Prussian troops reach Vienna
In this scenario, the Russians can be prevented from assisting the Austrians for numerous reasons, not least of which is Russian fury with Austrian betrayal during the Crimean War, still fresh in the tsar's mind (IMO, the Russians could even have been kept out in 1848, but thats another story).
An accommodation might be reached with France that prevents them from opposing it (cessions in Savoy, etc). But after more than 100,000 dead in the Crimea, France is still weakened at this point. Once France and Russia are neutralised, the British will be forced to accept whatever situation unfolds on the continent, thus avoiding a general war to contain the new Germany.
End result is the Conference of Vienna of 1861, where an 'Empire of Hungaria' with a Habsburg monarch, ruling over Hungary, Transylvania, Banat, Slovakia, Croat-Slovenia is created. Germany gets German Austria, Tirol and B-M, with Galicia-Lodomeria & Krakow awarded to Russia. All the powers sign off, and Wilhelm is crowned German Emperor with his investiture in the Hofburg or Schonbrunn, rather than Versailles.
A bit outlandish, but with the right kind of opportunists to take advantage of the situation, a wartime disintegration of Austria like that is possible in the 19C, leaving rump Austria to join Germany.