  Longitudinal spin diffusion of two pseudo-spin domains is studied in a
trapped $^{87}$Rb sample above quantum degeneracy, and the effect of coherence
in the domain wall on the dynamics of the system is investigated. Coherence in
a domain wall leads to transverse-spin-mediated longitudinal spin diffusion
that is slower than classical predictions, as well as altering the domains'
oscillation frequency. The system also shows an instability in the longitudinal
spin dynamics as longitudinal and transverse spin components couple, and a
conversion of longitudinal spin to transverse spin is observed, resulting in an
increase in the total amount of coherence in the system.
