The effects of a radio frequency electric field modulation on Stark tuned level crossings and saturated absorptions at 10/~m are investigated and compared with the dressed molecule theory. Applications are made to rotational quantum state assignment and electric dipole measurements. The dipole moments of NH2D in the ground and v2--1 state are measured to be ~"c --1.4579 + 0.0019 D, /,=1.08 _+0-13D.