Except for the King's Chapel (Boston) prayerbook, I can't think of a prayer book or hymnal, Universalist or Unitarian, that has the full psalter. What you get are partial set of psalms, and very often, you get tightly edited and amalgamated psalms "arranged for worship." As if the psalms were not intended for worship as-is.
I used to think this editorial hand was a bad idea, and then, editing the psalms myself, thought this action merely unfortunate. I couldn't shake the feeling that I was doing something intellectually dishonest by deciding which parts of the psalms were fit and which ones were not. I looked over to those Unitarian Universalists who had abandoned the psalms if not the role (in the form of "responsive readings") and guessed what might have aided the exodus.
The fact of the psalms in the daily prayers - from which Protestant Sunday worship is derived - is complicated and blended. The ancient urban laity had a thin but regular diet of psalms, enlisted for the people's praise of God. The ancient (and modern) monastics had a very full, even over-rich, diet of psalms, but then again they had the community and reserved time to engage with them. Our use is a blended inheritance of the two streams. We have some selectivity and some variety and quantity. Some praise and some contemplation. But I think we're asked to engage in too much contemplation with too much variety without the conditions (monastic life, reserved time to reflect) to help us deal with the psalms appropriately.
To be plain, some psalms are just too awful to be used in worship, at least the usual worship of non-monastics, whether lay or ordained.
Finally, I found a resource that put into words my feelings, and relieved me of the feelings of "cheating" on the psalms. George Guiver, of the Community of the Resurrection, writes in one of my favorite books (Company of Voices). First he considers the monastics and their inner journey, then in the following passage from page 156, he speaks to the blog-reading people and others "on the outside," thusly
The more we are caught up in the outer struggle of 'the world', the more [the Psalms] will need to be carefully chosen according to their ability to refresh and feed. Editing out the more difficult passages is often criticized on the ground that it is an avoidance of reality, but that degree of reality which includes breaking skulls of little children is best left to communities who know what to do with it. It is certainly inappropriate in many pastoral situations, or in fact anywhere where worshipers may not know how to handle such texts without puzzlement or revulsion.
As I plan for worship, personal and corporate, I intend to use the psalms, use them deeply, and use them selectively.