Surely Aragorn was too important for the future of Middle-earth for Gandalf to send him there for anything less than destroying the Ring, and with the weight of future Kingship on him why would he decide himself to enter with little chance of coming back?
Iolanthe, I don't think we have any information that specifically Gandalf sent Aragorn on this mission. Aragorn's message to Ecthelion only says, "Other tasks now call me." The "call" could have come from Gandalf, Elrond, the Northern Dunedain, etc., but I strongly believe the call came from within Aragorn himself. I think this was a duty he set for himself -- the final step into manhood. Whatever it was he accomplished, he returned from these perils armed with the confidence to accept Arwen's pledge. He was finally a Man, independent of his mentors, worthy of his lady love.