The YU Commentator has an article outlining the views of Rabbis Blau (quoting Rabbi J.B. Soleveitchik), Charlop and Lamm on Disengagement.
All three rabbis agree – as do the vast majority of halakhic authorities – that no halakha prohibits Disengagement, that soldiers should not disobey orders, and that the Israeli government has the right to make the decision to give away land.
This is the position of all major Sefardi halakhic authorities as well the vast majority of Haredi poskim.
The late Chabad-Lubavitcher Rebbe's contention that Shulkhan Arukh Yoreh Dayah 329 prohibits ceding land has never been taken seriously by halakhic authorities, and for good reason – the Rebbe's position makes no halakhic sense.
The same is true for those who claim that shleimut haaretz (wholeness or unity of the land) prohibits Disengagement. Why that mitzvah (itself in dispute) should halakhicly trump the several other mitzvot involved is not at all clear, which is why this position is not taken seriously in halakhic circles.
Should Israel cede land?
If I were Prime Minister (God forbid!), I don't know if I could do it. But the option – continual Israeli military presence to protect a handful of Jews in Gaza – is difficult to agree with, too.
In the end, it is the Israeli government's decision to make (and to live with) just as halakha mandates them to.
Let's hope – and pray – that the decision to Disengage is the correct one.