  We study the critical dynamics of three-dimensional ferromagnets with
uniaxial anisotropy by taking into account exchange and dipole-dipole
interaction. The dynamic spin correlation functions and the transport
coefficients are calculated within a mode coupling theory. It is found that the
crossover scenario is determined by the subtle interplay between three length
scales: the correlation length, the dipolar and uniaxial wave vector. We
compare our theoretical findings with hyperfine interaction experiments on Gd
and find quantitative agreement. This analysis allows us to identify the
universality class for Gd. It also turns out that the $\mu$SR relaxation rate
can be best fitted if it is assumed that muons occupy octahedral interstitials
sites within the Gd lattice.
