I have gotten as far as facing Ulfric and Galmar, after defeating the Stormcloaks. Tullius gives Ulfric his ultimatum. U and G decide to fight. I beat them down but neither dies. Ulfric crawls toward me with a little dialogue arrow over his head but no dialogue box appears. I have disabled any Mods I had installed, I had Steam check the cache -- it found one file needed replacing, and I started the game from the last Autosave. Still Ulfric crawls.

Stormcloaks never die!

Wrong forum, dude.

It sounds like they've not lost their "essential" tag - maybe you skipped part of the quest?

I know when I joined the Stormcloaks Ulfric gave me the option to take out the General or let Ulfric himself do it. I chose the former only because I wanted to know what it felt like to kill a Cylon but if the same option is available with the Imperials maybe you can try letting Tullius do the dirty work.