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that's a long job-- pretty much have to remove the rear diff cover just to remove the c-clips that hold the yokes in place- and in order to remove the cover, you'll have to remove the transverse spring then remove the 4 bolts that hold the cover to the upper rear end crossmember. not a fun job-- what makes you suspicious they are worn?
Look for scoring on the side of the diff where the yokes might rub on the case.
I had this nasty experience with an almost-purchased 79 4-speed (I'm trying to keep up with Paul Borowski) that needed an engine, trans and diff rebuild. You could see the circular scoring on the side of the diff because the yokes were moving freely in/out of the case.