  We observe the translational and rotational diffusion of dimer tracer
particles in quasi-2D colloidal samples. The dimers are in dense samples of two
different sizes of spherical colloidal particles, with the area fraction
${\phi}$ of the particles varying from dilute to nearly glassy. At low ${\phi}$
rotational and translational diffusion have a ratio set by the dimer size, as
expected. At higher ${\phi}$ dimers become caged by their neighboring
particles, and both rotational and translational diffusion slow. For short
dimers we observe rapid reorientations so that the rotational diffusion is
faster than translational diffusion: the two modes of diffusion are decoupled
and have different ${\phi}$ dependence. Longer dimers do not exhibit fast
rotations, and we find translational and rotational diffusion stay coupled for
all ${\phi}$. Our results bridge prior results that used spheres (very fast
rotation) and long ellipsoids (very slow rotation).
