In this article, we consider the dynamics of short chains in frozen polymer networks, at different degrees of swelling. The crosslinked networks are bimodal, at monomer densities ranging from 0.51 in the dry state to 0.098 at the maximum degree of swelling. The 3D bond-fluctuation method is used to simulate the polymers, and we use a Monte Carlo method for monomer moves which has been presented in an earlier publication (Nedelcu and Sommer, J Chem Phys 130:204902, 2009). Crossover scaling functions for the terminal relaxation times tau (d) and diffusion coefficients D of the short chains are observed to approach de Gennes' reptation predictions, similar to what has already been observed for long chains diffusing in polymer melts. However, the mean square displacement of a middle monomer of a chain in high-density networks shows only two regimes, first subdiffusive with a scaling exponent close to 1/4, followed by a smooth transition to free diffusion at longer times.