This strikes me as quite a thought-provoking question. Throughout Steinbeck's novel we find references to rabbits. Most of these references come from the little house that George and Lennie dream of having someday.
The part of their dream that most appeals to Lennie is taking care of the rabbits. Whenever George wants to punish Lennie, he threatens to take this job away from Lennie: “But you ain’t gonna get in no trouble, because if you do, I won’t let you tend the rabbits.” Whenever Lennie gets in trouble, he worries that George will not allow him to tend the rabbits.
So, on one level, the rabbits represent for Lennie the best part of their own personal utopia. On another level, the rabbits represent a mechanism by which George tries to control Lennie's behavior.
The rabbits also remind us of the soft things that Lennie likes so much. Unfortunately, Lennie's affinity for soft things gets him into trouble, as it did when he touched the woman's dress in Weed and when he touched the hair of Curley's wife. Steinbeck's word choice when George describes the reaction of the woman in Weed is quite ironic: "Well, that girl rabbits in an’ tells the law she been raped. The guys in Weed start a party out to lynch Lennie."
Lastly, we might also suggest that the rabbits remind us of Lennie's simplicity and the simplicity of what it would take to make Lennie happy. In contrast to human beings, rabbits seem like simple creatures. Lennie Small was a person with a simple mind who had a simple dream: he want to tend rabbits on a little farm with his friend George.