WordNet sense 1 (vibrating slightly and irregularly; as e.g. with fear or cold or like the leaves of an aspen in a breeze):

shivering, trembling

WordNet sense 2 (inclined to shake as from weakness or defect):

rickety, wobbly, wonky

WordNet sense 3 (lacking stability or fixity or firmness):

unstable

WordNet sense 4 (not secure; beset with difficulties):

precarious

WordNet sense 5 (lacking in security or safety):

insecure