hi you may of overfilled the box or wrong grade of oil as far as i know you use 7 quarts of oil to fill.the box should be warm before you drain it then take the bottom of gearbox sump off clean it off ,then take oil filter out of sump put new one in new sump seal bolt back up then fill.before you drive it work it up and down the gears a few times should be ok to drive then.

p.s dont take the magnets out the bottom of sump and check they are in the 2 little wells they sit in as they collect metal in oil from wear.

What oil did you use and how much did you add?