"Shimon and Levi are brothers;
Their weapons are tools of lawlessness.
Let not my person be included in their council,
Let not my being counted in their assembly.
For in anger, they slew a man,
And they uprooted an ox.
Cursed be their anger so fierce,
And their wrath so relentless.
I will divide them in Yaakov,
And scatter them in Israel." (v.5-7)
These are Yaakov's words to his two sons, as he lies dying on his death-bed. This is his parting message and it is quite the indictment! Yaakov curses Shimon and Levi, or more accurately, he curses their anger and violence. He tells them that they should be scattered and dispersed in Israel. This is a damning condemnation.
If the issue is so clear to Yaakov, then why does Yaakov remain so silent in our parasha? Why does he speak only years later? If Yaakov seems so sure that their acts were harmful, immoral, then he should have said something at the time that the atrocity occurred, not to wait forty years!
One approach might suggest the following resolution. The true moral voice is Bereshit 49 in which Yaakov's condemns Shimon and Levi. As for his silence at the time, it might simply follow the proverb that we know from Pirkei Avot:
"Do not (try to) placate a friend in his moment of anger;
Do not (attempt to) comfort him while his dead relative lies before him." (Avot 4:23)
In the heat of the moment, one cannot discuss the moral implications. Shimon and Levi will not be open to Yaakov's ethical rebuke. Years later, when the passions have calmed, Yaakov takes his final moments to sound a clear moral message for all time, putting his personal moral opinion before his sons, and with the writing of the Torah, before all of Am Yisrael. There is no ambiguity here, no hesitation. One simply has to find the correct moment in which to issue a rebuke of this magnitude. Yaakov knew that he had to wait for such an opportunity. He found it only on his deathbed.
So this is one possibility.
Rav Yehudah Shaviv (a prolific writer, who lives and teaches in the Gush,) saw the dichotomy differently. He focuses upon Rashi's comments to verse 6:
"For in anger, they slew a man: This is Sh'chem and Chamor…
And they uprooted an ox: They sought to uproot Joseph, who is called an Ox (Deut 33:17)"
Here Yaakov draws a direct connection between the Sh'chem episode and the sale of Joseph. At the time, Yaakov saw certain justifications for the actions of Shimon and Levi. He didn't condemn their actions. Maybe there is a certain guilt to a town that harbours criminals, not bringing them to justice.
But after the Joseph affair, Yaakov sees things differently. Once you have murdered, a further murder seems less intimidating. Murder becomes a realistic option, not an unthinkable crime. The fact that Shimon and Levi had killed Sh'chem allowed them to consider murdering Yoseph. They had learned that they could channel insult and indignation in the direction of violence, homicide. They began by killing criminals, they tried to kill their brother.
At the time, their actions seemed justified. In hindsight, the violence in Sh'chem came to be viewed as the moment in which the brothers lost their innocence, they became jaded and morally clouded. It was indeed, a crime.
Yaakov realises that a direct linkage connects the Sh'chem killings, with the attempted murder and subsequent sale of Joseph.
According to the first explanation, Jacob was correct in not issuing a rebuke in our parasha. According to the second argument, Yaakov's lack of response was a total lack of moral guidance, which was in error. The devastating effects of his silence became clear only much later.
 At least 40 years, and probably longer. 22 years of Yoseph's exile and a further 17 years during which Yaakov lived in Egypt. There are at least 40 years between the Sh'chem affair and Yaakov's death.
 Rashi in Bereshit 42:24 suggests that Shimon and Levi were the prime instigators in suggesting the option of murdering Joseph.
 If I recall correctly, Rav Shaviv intimated that had Rabbis been more forthright in absolute condemnation of Baruch Goldstein's massacre of Arabs, the legitimacy would have never been found for Yigal Amir's assassination of Yitzhak Rabin. Or – if we are quiet when a Jew kills a non_jew (Shchem), then the next stage will be a Jew killing a Jew! (Yoseph)