Oops! Sorry for the late, late reply, but here goes, according to my older Rancho catalogs:
Rancho made 6 different leaf springs for the '73-'87 trucks and up to '91 Blazers and Subs. Some are still made and some not.
1. #86201 (.75-1.5 inch lift), spring rate is 340, 3 leafs per spring.
2. #86202 (2.5 inch lift), spring rate is 450, 5 leafs per spring. (Very popular 2.5" springs)
3. #86206 (4 inch lift), spring rate is 520, 5 leafs per spring. (Very popular 4" springs)
4. #86207 (6 inch lift), spring rate is 680, 9 leafs per spring. (No longer made)
5. #86210 (2.5 inch lift), spring rate is 348, 9 leafs per spring. (Pre-runner springs)
6. #40054S (4 inch lift), spring rate is 672, 5 leafs per spring. (Popular stiff 4" springs)
There you have the Rancho '73-'91 springs. Yours would be the #86210's. Does yours have 9 leaves in the pack?
Rancho also made 2 rear springs. Their info is:
1. #86209 (4 inch lift), spring rate is 287, 6 leafs per spring.
2. #86211 (2.5 inch lift), spring rate is 230, 6 leafs per spring.
These are the Rancho rear springs only available in 52" spring-eye-to-eye.