[redz 1970 K5]

Your blazer probably has 3.42s or 3.73s. Those are probably the most common ratios. I would say 3.73s for a base 4x4. Look below, if you look at your RPO codes in your glovebox and compare it to the axle ratios below, you will know what you have.

Here's the axle ratio RPOs:

GT4 - 3.73 Rear Axle Gear Ratio

GT5 - 4.10 Rear Axle Gear Ratio

HC2 - 2.76 Rear Axle Gear Ratio

HC4 - 4.56 Rear Axle Gear Ratio

HC8 - 3.21 Rear Axle Gear Ratio

HE3 - 3.07 Rear Axle Gear Ratio

HO4 - 4.11 Rear Axle Gear Ratio

As far as testing for limited slip , that is a good question. I think you are right- if it's a limited slip, and you just turn the wheels manually, it will feel like an open diff.

My only recommendation would be to look at the trucks RPO codes in the glovebox (if they are still there), and look for the locking rear diff RPO code. You are going to be looking for:

G80 - Locking Rear Differential