I replaced mine this past winter. I don't read or measure in the same language as a mechanic or machinist but from what I read and found out somewhere around an 1/8" (.o125) or a bit less is about the max. They wear out big time on the inside and you get more lateral (vertical) movement in the wheels when cornering. If they wear too much the seal shields or whatever they are called could be hitting the diff housing under corning pressure.
I'm by far no expert on this stuff so maybe someone that knows more can give you a better idea.

I replace all the u-joints (6) on my Vette last winter. The scraping and other drive-line noises went away after that. Make sure you get a 4 wheel alignment after you replace these and other suspension/drive-line pieces. I'm sure you will be happy with the results...the rest of the season.

I have never seen any published specs on the side yoke clearance/end play...not even in the factory service manuals. I think around .010"-.015" is about max(my opinion). Any more than that will effect the rear camber adjustments as the suspension moves up/down in turns/corners.

[QUOTE=gellisvette] I'll know more when I open it up this weekend. I'll try to keep up up to speed. If you guys want?[/QUOTE]

By all means. let us know what you find in there.
Mine made that chattering noise taking off from a dead stop and turning. I added some oil and posi additive and it stopped. I opened mine mainly because the cover and side seals leaked and I was doing a new spring. the oil looked like a metallic paste. New seals, side yokes, clean oil and she runs like a dream now.
Jeffm2009-08-20 17:45:27

I haven't gotten mine in to the shop yet so the jury's still out.

My vote is that the positraction clutches are sticking.  I have had that problem.  Change the oil and that may solve the problem.  That is the easiest and the least expensive.  Not that easy to do though because there is no drain plug.  Many people put one in.  The other solution is to suck it out.  Make sure you put in a good quality positraction oil and the additive and your problem should go away.  The answer to your differential side yoke question regarding how much outward movement.  Very little!  If you can feel it, the yokes are probably worn.  A clue is if when the car is jacked up, if the top or bottom of the tire has more than 1/4" in-out movement, the yokes are worn.

Keep in mind, if the yokes are worn, that can also make the posi clutches act up.  Mine does that.  I have the new yokes in a box to go in when I get the time.