My department is not very supportive when a colleague leaves

[fizzycist]

I have found the conventional wisdom that one needs a letter from a departmental colleague when applying does not really hold, and it has not prevented me from getting interviews at highly ranked and prestigious institutions where it's clear why one would leave the current position for a new one. Mind you, that doesn't prevent someone from the search committee contacting one of your colleagues off book to see if there are any skeletons in the closet. I've been the recipient of such a call one time when a senior colleague of mine applied somewhere else. People on the search committee are more likely to be concerned if you're applying to a lower ranked or less desirable position.

Me too. And I have reviewed applications from many faculty looking to move and they rarely (never?) have had letters from folks at their current place. And that is considered completely normal by the other committee members and some of them still got interviews too.

If it trying to get the dirt on someone it's not gonna be from someone the applicant selected.