While the Jarl’s help is conditional on your getting a truce, word has been spread throughout the whole of Skyrim that you are, in fact, the Dragonborn- so while people may not instantly recognize you as such, when you claim to be so, people will be inclined to listen to you and believe you. This is going to give your argument some weight, but you’re going to need more than that for the sake of calling a truce. Above all, you need neutral ground, where both Tullius and Ulfric are willing to go without extreme suspicion of a trap. This makes your next goal requesting the help of the Greybeards.
As generally apolitical parties with no direct interest in the civil war of Skyrim, the Greybeards- or more specifically Arngier- are an ideal group to both moderate and host the truce meeting, so that you can ensure things remain civil. Some players will prefer that things remain fair as well, but leaning the peace talks or keeping them neutral is really up to you. While you can favor one side or the other in the conflict, it doesn’t actually affect how they react to you, just what territories and advantages are held by which side- but we’re not at that point yet.
Knowing that the Greybeards hold the only truly neutral territory, you need to head up the mountain to talk to Arngeir about this truce meeting you want to call. The Greybeard is considerably less than enthusiastic about the prospect: the Greybeards are meant to be wholly apolitical and a truce meeting is as likely to be political as dragons are to be big or the ocean is to be wet. However, as it is the only method you have of convincing Whiterun’s Jarl to allow you use of the palace, he agrees- but you’re the one who’s going to have to convince General Tullius and Jarl Ulfric to actually meet with one another.
This is where things start to get a bit complicated again. You’ve already been to Solitude for the sake of earlier quests in the main quest line, and you’ve been to the gate of Windhelm thanks to your good friend Delphine, but if you’ve been fighting in the civil war already things can get dicey as far as reaching one or the other leader, particularly if you’ve been doing ‘extra-curricular’ wartime activities such as attacking Stormcloak or Imperial guards (depending, again, on which side you’re on). Still, the conversations themselves are simple enough once you get to the two men. If, however, you haven’t joined either side, things get a bit sticky. If you have picked a side, move on to Part 16d, but if not, continue to Part 16c.