Section the grapefruit without the membrane, saving the juice in a small bowl. From the leftover membrane, squeeze and reserve as much juice as possible. Set aside the grapefruit sections.
To make the Grapefruit Shallot Vinaigrette, add all of the vinaigrette ingredients into a Mason jar. Close lid tightly and shake.
Just before serving, pit and thinly slice the avocados. Assemble the salad and toss with the vinaigrette.