וַאֲנִי נָתַתִּי לְךָ שְׁכֶם אַחַד עַל אַחֶיךָ - bereshis 48:22. Yaakov previously gave Yosef an additional article of clothing, which spurred the brother's hatred of him (Yosef). Why would Yaakov go out of his way to say that this gift of 'Shechem' was one more than his (Yosef's) brothers once again?
Derishos Shabbos - The Apta Rav - page 231 - top left says that Shechem was given as payment for Yosef taking care of Yaakov's burial, which the other brother's were unable to do.
ושכם הרי אינו נותנו לו בחנם רק עבור מה שמטריח אותו בקבורתו מה שאין סיפק ביתר הבנים לעשותו
In my answer to another question, I summarized the approach of R' Goldvicht ZT"L as to why Yosef did not reveal himself to his brothers. The climax of that answer is that when Yosef saw that Yehuda was willing to put himself on the line to save Binyomin, he understood that the perceived jealousy that had existed between the other brothers and the sons of Rachel had been overcome (see there for the whole story). Based on this, R' Goldvicht also explained how Yosef could show favoritism to Binyomin by giving him 5 sets of clothing while giving the other brothers only 1 each.
The same could be applied to Yaakov - it had been demonstrated that the jealousy that had existed was something they had overcome, and Yaakov felt it was now actually safe to treat him differently.
Pirkei D'Rabbi Eliezer, Perek 38, states:
ויצאה דינה לראות בבנות הארץ... ושכב עמה והרתה וילדה את אסנת
And Dinah went out to gaze upon the daughters of the land... and [Schechem] slept with her, and she became pregnant, and she gave birth to Asenath.
With all the people of Schechem dead, the heir to the throne is Asenath. When she marries Yosef, the deed of the land is passed into his family. Thus, Ya'akov is merely deciding not to confiscate and redivide Yosef's property.