Did you put the third in or did a shop...if the shop set the gears up and also put the third back in I would definitly take it to them.  Have you been easy on the gas?  http://www.marlincrawler.com/htm/diff/ringandpinion.htm  If you didn't do what these directions say (before you changed the oil) you may have already burned the oil...causing the gears to possibly be no good.  Also...is your pinion in the back angled?  If it is I think you need to put more oil in than what it would show...the filler won't be accurate anymore.  To sum it all up...take it back to the guy that did the work.