He's right. You'll have to take the L/U apart anyway, so half the job is already done. Their's a pair of back to back seals under the w/p for the driveshaft, another behing the prop on the shaft and maybe a third on the shift shaft. Very easy to do: pry the old ones out (a seal puller may help here), discard and insert new ones by gently pushing them in with a socket and a hammer. It will cost you no more than 50$.