I'm in the process of replacing both rear wheel (axle) bearings on my 86 Sammy and there is a fair amount of play in the 3rd member. How much play is too much? 

What do you mean by play?  Do you mean movement in the pinion before the gear engages or is the flange loose and move around?

Hi Drone. I mean movement (rotation) of the pinion before the gear engages. Mine has quite a bit, maybe 1/4 turn each way, forward & reverse. The flange seems solid.

Do you have a lunchbox locker?  If so, that it pretty much normal. 

Even stock, the diffs do have quite a bit of play.  90 degrees seems pretty excessive, but if it isn't groaning or grinding, I wouldn't worry about it. 

Thanks for your reply Jonny. It's stock. I guess I'll worry about it later.

If it hasn't been touched I wouldn't worry about it.  Unless you want to pull the third member out and check, which isn't that hard if your axles are already out.