  The spectral index $s$ of particles diffusively accelerated in a relativistic
shock depends on the unknown angular diffusion function $\mathcal{D}$, which
itself depends on the particle distribution function $f$ if acceleration is
efficient. We develop a relaxation code to compute $s$ and $f$ for an arbitrary
functional $\mathcal{D}$ that depends on $f$. A local $\mathcal{D}(f)$
dependence is motivated and shown, when rising (falling) upstream, to soften
(harden) $s$ with respect to the isotropic case, shift the angular distribution
towards upstream (downstream) directions, and strengthen (weaken) the particle
confinement to the shock; an opposite effect on $s$ is found downstream.
However, variations in $s$ remain modest even when $\mathcal{D}$ is a strong
function of $f$, so the standard, isotropic-diffusion results remain
approximately applicable unless $\mathcal{D}$ is both highly anisotropic and
not a local function of $f$. A mild, $\sim 0.1$ softening of $s$, in both 2D
and 3D, when $\mathcal{D}(f)$ rises sufficiently fast, may be indicated by
ab-initio simulations.
