I never saw how much shock/spring travel you have. More travel should lend to a lower rate spring. For my $.01 (can't afford $.02), here's my calculations.

LR- 737
RR- 845

Total rear weight is 1582 pounds. As stated above, deduct the unsprung weight of the rear axle, wheels and tires, etc. I subtracted 250 pounds here and ended up with 1332 pounds rear sprung weight. Divide that by two (two tires) and I get 666 pounds to be supported per side.

If you have seven inches of total travel, maybe you would want to compress the spring four inches per Cab's 2/3 rule-of-thumb. 666 pounds/4 inches = 166.5 pounds/inch spring rate.

If you have ten inches of total travel, maybe you would want to compress the spring six inches. 666 pounds/6 inches = 111 pounds/inch spring rate.

Q.E.D.

That's my theory anyway. Check some of the chassis books and see what they say. You may have to test a few spring rates and shock settings to find a good setup? Hopefully this can get you in the ballpark.