Axle splines on an 82 D150?

I have an 82 D150 two wheel drive, slant with 833 OD. The rear end is an open 8-1/4, about 3.55 gears. I'd like to add a sure-grip to this, but I can't seem to find out what will fit. I understand that some of these rears had 27 spline axles, and others had 29 splines. Does anyone know what splines I have so I don't have to take it apart to figure it out?

The next question is, would any 8-1/4 SG with the right spline count fit? I think I found one in a mid-to-late 90's Dakota but again, I can't seem to find solid info on this rear end either.