I'm going to go from obvious to more detailed!
"Ubar" refers either to the war chief of a city or to the permanant leader of a wagon camp, whereas a "High Jarl" is the head honcho in a steading, villiage, or mead hall of Torvaldsland.
Furthermore, except in the case of the Wagon Peoples, a Ubar rules only in times of war; otherwise, it is an Administrator and a panel of advisors who leads a city. (Port Kar and the Council of Captains being the exception.) It is my understanding that a High Jarl is a lifetime ruler, since he's much more like the Lord/man of the house.