Hard to evaluate. Marino always came off as a "nice guy", not really making any frustrations public. Not sure if he pushed hard or not for getting a RB to help him, though one would imagine that he saw what happened with Elway when the Broncos got Terrell Davis.
In his playing days, the Dolphins used 3 first round draft choices on RB, a couple of seconds (though both were in 1999 with JJ Johnson and Rob Konrad), three thirds, and a couple of fourths.
Maybe someone can recall if the Dolphins sat out any major free agent signings of RBs from 1984-1999. I seem to recall a lot more guys being willing to leave their long-time first teams after Reggie White signed with GB in 1993.
Also not sure what could have popped up on trades. We all remember the Sammie Smith for Bobby Humphrey trade, which bombed for both the Dolphins and Broncos. The Marshall Faulk trade didn't happen until Marino's final year in the NFL. I guess the Dolphins could have made a play for Jerome Bettis.