1990 Sierra Rear Differential question

[Sigma.40]

1990 GMC Sierra SLX 1500 5.7L 4x4. Last night I decided to check my rear diff fluid and saw that it was completely black and metallic silver. It was changed like 12-18 months ago so it shouldn't be that bad.

 

I pulled the cover off and cleaned all that stuff out and noticed that one of the spider gears moved up and down on that little shaft about 1/2" but the gears all looked good otherwise. I re-filled it with fresh fluid but obviously something is screwed

 

It has a 10 bolt cover, which rules out the 9.25" diff. Most sites list an 8.5" rear but some list an 8.6". RockAuto also lists parts for an 8.25". There are no numbers anywhere. Any ideas what differential I have? How do I determine if I have an 8.5" or an 8.6"? 

 

I don't have a ton of experience with rear axles other than replaces a few complete solid axle assemblies so I haven't done much differential/rear bearing repair. Thanks for any help

 

[Rme2513]

I believe they are all 8.6 and the spider gears need to be replaced I bet the new ones want even look the same when you get them in I had to do the same on a 84 k5 mine just broke not to hard to change out but be aware  that bolt / pin that holds the center pin in will more then likely break when you go to take it out 

[Logan Lanfear]

Could always take it apart and measure.  It has to come apart anyway and I wouldn't drive it with that much slop in the system.  Sounds like the axle needs a rebuild.  Might be cheaper to get a junkyard axle.