Check rear block height, are you are measuring center of axle to top of fender well ? There is not a product that compensates for 4" difference with a puck already installed thats not custom, 4" lift springs maybe but your guessing.

 

I'll have to do it when I get home but what do I do if there are custom blocks in the rear to level the truck?

 

remove them and put the stock blocks back in and you should be close to level now with your puck. Should be a dime a dozen at a pick a part

 

If I leave the rear blocks as they are what do I need to do to level the front

 

not sure, get the measurements and post or PM me we can come up with something

 

Get some pictures and see if you can measure how thick the rear blocks are.

This is a 2x4 truck too, I need to keep reminding myself.