The Carli 2. 5 system would be a great choice, as would a setup from Thuren at Thuren Fabrications. Another option would be the Lorenz 2. 5 setup. I think the main difference between these kits is the spring rates used. It seems hard to find concrete numbers, but I think Thuren's would be the softest, followed by Carli, and then Lorenz (although Sean is offering a softer ride coil now too).
Oh, another difference just came to mind. If you're running 37's, you may want the Carli replacement rear spring pack, as they offset the center button some to clear the larger tires. I have the Lorenz DP springs, and while the ride is great, I had to preload a heavy set of traction bars to gain clearance for the 37" Toyos.
On any of these systems, the Carli Long Travel air bags really help out if you do any towing or haul the bikes in the bed of the truck. I'd recommend the poly bumps that Thuren carries for the front as well... cheap insurance that will let you get by with a little more without bending an axle housing.
--Eric