What is Tavor?
Bi'ur ha'Gra: It is Mount Tavor.
Malbim #1: It is Kislos (Rashi 12 - on the slope of Har) Tavor, in Zevulun.
Malbim #2: It is a city, like it says in Divrei ha'Yamim (1:6:2) "Tavor v'Es Migrasheha."
In which Shevet was Tavor?
Bi'ur ha'Gra: It was in Yisachar, but Aznos Tavor was in Naftoli, and Kislos Tavor was in Zevulun; it begins in Naftoli, and enters Zevulun via Yisachar, bent.
Where were Shachatzimah and Beis Shemesh?
Bi'ur ha'Gra: They were from Tavor until the Yarden from below to above 1 .
Perhaps 'from below to above' means from west to east. In all three maps that the Vilna Gaon drew of Eretz Yisrael (at the end of Sefer Eretz Yisrael li'Gevuloseha Saviv), east is at the top. (PF)
What are "Totz'os Gevulam"?
Malbim: Its borders on two sides, i.e. north and south, ended at the Yarden. Below we will explain that it was not truly until the Yarden, rather, the borders met there, like in Yosef. What was truly at the Yarden was of Naftoli.