go to filthymotorsports.com
There are a lot of videos on choosing shock length, spring rates, set ups, pre-load, etc. I bought my shocks from them. There is a calculator for spring rates too, plug in your dimensions and corner weight, angles, link lengths, etc. Shocks can drive you crazy figuring things out which means wasted money as I can attest. Longer shocks give you more range of adjustment. Lets say you have 16 inch travel shocks with dual rate 16 inch springs, 16 inch long spring on top, 16 long spring on bottom. If your ride height is too high you can swap out one of the 16 inch springs for a 14 inch and drop the ride height two inches. You can also swap a 16 inch spring for an 18 inch to raise the ride height as I did. Good luck.
If your not spending money on your truck it's either not finished, not broken or your brain dead because you dont want to change something.