If your replacement transmission is a used trans, then replace the axel seals. The seals a relatively cheap and easy to replace when the trans is out of the car. The outer part of the seal that contacts the trans case should be coated with a thin coat of anerobic sealer. The rubber seal itself should come with a bead of grease around it.
Be careful inserting the axels into the seals so you do not cut or otherwise damage the seal. Check the axels for any burrs or rough spots where they are inserted into the seal. Make sure they are clean and apply a light coat of trans fluid just before you insert them into the trans.
