The Collaborative International Dictionary of English v.0.48:

Tremulous \Trem"u*lous\, a. [L. tremulus, fr. tremere to
   tremble. See Tremble.]
   1. Shaking; shivering; quivering; as, a tremulous limb; a
      tremulous motion of the hand or the lips; the tremulous
      leaf of the poplar.
      [1913 Webster]

   2. Affected with fear or timidity; trembling.
      [1913 Webster]

            The tender, tremulous Christian.      --Dr. H. More.
      [1913 Webster] -- Trem"u*lous*ly, adv. --
      Trem"u*lous*ness, n.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

tremulous
    adj 1: (of the voice) quivering as from weakness or fear; "the
           old lady's quavering voice"; "spoke timidly in a
           tremulous voice" [syn: quavering, tremulous]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "tremulous":
   afraid, agitated, aguish, all shook up, all-overish, anxious,
   aquiver, aspen, bashful, blurred, breathy, chattering, choked,
   choking, cowardly, croaking, diffident, doubtful, drawling, drawly,
   dysphonic, faltering, fearful, fearing, fearsome, fidgeting,
   fidgety, fluttery, frightened, goosy, guttural, harsh, hawking,
   hesitant, hoarse, in a quiver, in fear, inarticulate, indistinct,
   jittery, jumpy, lisping, mispronounced, mousy, muzzy, nasal,
   nervous, palpitating, palsied, quaking, quavering, quavery,
   quivering, quivery, rabbity, scared, scary, shaking, shaky,
   shivering, shivery, shook up, shrinking, shuddering, shy, skittery,
   skittish, snuffling, startlish, stifled, strangled, succussatory,
   succussive, thick, throaty, timid, timorous, trembling, trembly,
   tremulant, trepidant, trigger-happy, twangy, twitchy, twittery,
   unsteady, unsure, velar, vibrating, wavering, wobbly, worried