A couple of gearing questions

[TugOwar]

Good day all,

1) A few days ago I had the back of my 68 3/4 GMC off the ground adjusting the rear brakes and decided to clean up the little tags on the 10 bolt cover and see what they said. One tag bolted on the right side say "4:10", I know what that means.

But the tag bolted on the left says "use limited slip gear lube only." Now I know that limited slip is different from posi traction but I don't recall seeing "posi-trac" gear lube in the parts store. So I'm wondering if just because it calls for limited slip lube does that necessarily mean it's a limited slip rear end, or does a posi-trac also take limited slip lube? I don't know if it means any thing but I noticed while adjusting the brakes that if I turn one wheel, the other wheel turns in the same direction, forward or back.

I hope all that isn't too confusing. From what I've written is it possible to tell if I have limited or posi rear?

2) I know this question has been asked a blue zillion times but for whatever reason I can never find anything using the search function, I don't know what I'm doing wrong, anyay..Please forgive the repetitive question.

My speedometer is wrong, too fast actually. I think I remember reading that this is because one of the speedo gears in the tranny needs to be changed, either the drive gear or the one that slides into the tranny. But I don't which one, and I don't know how to determine which to replace it with or where to get a replacement.

Here is what I know:
4:10 gears
10.50x31x15 rear tires
TH350 tranny
Factory Speedometer

What other info do I need to get my speedo correct? Any help would be greatly appreciated.

Thanks,
Tug