  We have studied (Tb$_{1-x}$La$_x$)$_2$Mo$_2$O$_7$ pyrochlores by neutron
diffraction and $\mu$SR at ambient and under applied pressure. Substitution of
Tb for La expands the lattice and induces a change from a spin-glass like state
({\itshape x}=0) to a non collinear ferromagnet ({\itshape x}=0.2). In the
ferromagnetic structure, the Tb moments orient close to their local anisotropy
axis as for an ordered spin ice, while the Mo moments orient close to the net
moment. The $\mu$SR dynamical relaxation rate shows a cusp-like peak at the
Curie temperature T$_C$ and a broad anomaly at T$^\star$$<$T$_C$, suggesting a
second transition of local or dynamical nature. Under pressure, the long range
order breaks down and a spin glass-like state is recovered. The whole set of
data provide a microscopic picture of the spin correlations and fluctuations in
the region of the ferromagnetic-spin glass threshold.
