  We study the Knapp-Stein $R$--groups for inner forms of the split group
$SL_n(F),$ with $F$ a $p$--adic field of characteristic zero. Thus, we consider
the groups $SL_m(D),$ with $D$ a central division algebra over $F$ of dimension
$d^2,$ and $m=n/d.$ We use the generalized Jacquet-Langlands correspondence and
results of the first named author to describe the zeros of Plancherel measures.
Combined with a study of the behavior of the stabilizer of representations by
elements of the Weyl group we are able to determine the Knapp-Stein $R$--groups
in terms of those for $SL_n(F).$ We show the $R$--group for the inner form
embeds as a subgroup of the $R$--group for the split form, and we characterize
the quotient. We are further able to show the Knapp-Stein $R$--group is
isomorphic to the Arthur, or Endoscopic $R$--group as predicted by Arthur.
Finally, we give some results on multiplicities and actions of Weyl groups on
$L$--packets.
