If $E/F$ is a quadratic extension $p$-adic fields, we first prove that the $mathrm{SL}_n(F)$-distinguished representations inside a distinguished unitary L-packet of $mathrm{SL}_n(E)$ are precisely those admitting a degenerate Whittaker model with respect to a degenerate character of $N(E)/N(F)$. Then we establish a global analogue of this result. For this, let $E/F$ be a quadratic extension of number fields and let $pi$ be an $mathrm{SL}_n(mathbb{A}_F)$-distinguished square integrable automorphic representation of $mathrm{SL}_n(mathbb{A}_E)$. Let $(sigma,d)$ be the unique pair associated to $pi$, where $sigma$ is a cuspidal representation of $mathrm{GL}_r(mathbb{A}_E)$ with $n=dr$. Using an unfolding argument, we prove that an element of the L-packet of $pi$ is distinguished with respect to $mathrm{SL}_n(mathbb{A}_F)$ if and only if it has a degenerate Whittaker model for a degenerate character $psi$ of type $r^d:=(r,dots,r)$ of $N_n(mathbb{A}_E)$ which is trivial on $N_n(E+mathbb{A}_F)$, where $N_n$ is the group of unipotent upper triangular matrices of $mathrm{SL}_n$. As a first application, under the assumptions that $E/F$ splits at infinity and $r$ is odd, we establish a local-global principle for $mathrm{SL}_n(mathbb{A}_F)$-distinction inside the L-packet of $pi$. As a second application we construct examples of distinguished cuspidal automorphic representations $pi$ of $mathrm{SL}_n(mathbb{A}_E)$ such that the period integral vanishes on some canonical copy of $pi$, and of everywhere locally distinguished representations of $mathrm{SL}_n(mathbb{A}_E)$ such that their L-packets do not contain any distinguished representation.
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