I have a 1984 Chevrolet S-10 Blazer 4x4. The rear leaf springs are riding on the stock 5" snubbers. Now I was told I have 3 pack leaf springs with an over load spring on the bottom of the pack.  

I was in the wrecking yard a few days ago & looked at another Blazer like mine & it had 4 + 1 overload leaf springs on it but the u bolts look as if they have been changed maybe.  

What I mean by "overload" is the bottom leaf is semi flat & wide on the ends. Since I don't know much about springs, other than what I read on your site here. 

Which one, 1,350lbs or 1,750 lbs would be correct for my application? Wondering what might happen if I use the heavy duty on this application if that is not the correct stock replacement? - Paul

 

The "overload" is technically called the "Second Stage". The top curved leaves are the "First Stage." 

The spring carries it's normal load on the first stage. As the load increases the first stage deflects and begins to make contact with the single leaf second stage. 

As the first and second stages come together, the spring rate automatically increases which "stiffens" the spring. This allows the spring to carry more weight while reducing the vehicle sag. 

In other words, a 2 stage leaf spring is a variable rate spring. 

Any of the 3 springs shown for your Blazer, the 2/1 1,100 lb, or the 3/1 1,350 lb or the 4/1 1,750 spring will work fine. Because of their design all 3 springs will sit the vehicle at the same stock factory ride height. The difference will be the carrying capacity and the ride quality. 

The lighter the spring, the less capacity and the softer the ride. - Mike