Are you talking about a PENSION plan, or a purely discretionary Profit Sharing Plan? At any rate, I think you want DOL regulation 2509.94-3, (c).
P.s. - the (c) is if it as a PS plan. If you are actually talking about a DB plan, then it would be paragraph (b). But as to whether a CD could be considered "cash" - I'd opine, for what little it is worth, that it's verboten. I lean towards conservatism on such issues. Ultimately an ERISA counsel question as far as I'm concerned.