What is the difference between a single leaf Add-A-Leaf and a 3 leaf progressive add-a-leaf?

The single leaf Add-A-Leaf (AAL) consists of a thicker leaf that has an arch that is greater than that of the factory leaf packs. When the AAL is added to the leaf packs the AAL will force more arch out of the factory packs which in part helps to accomplish a portion of the rear lift. Lift is also accomplished through the addition of steel added to the pack which does raise the spring rate thus allowing the factory leaf pack to support more rear weight. 

The 3 leaf AAL consists of thinner leafs with minimal arch. Having minimal arch the 3 leaf AAL fits against the factory packs nicely vs. trying to re-arch them like the single leaf AAL does. This along with the leafs being thinner and having a pyramid stack design (long to short leafs which engaged as the packs are flexed during up travel), most will say the ride is improved over a single leaf AAL. Unlike a single leaf AAL, a good portion of the lift is accomplished through the overall thickness of the 3 leafs. However, like the single leaf AAL the 3 leaf AAL does raise the spring rate and this can contribute to a firmer ride over the stock leaf packs. 

Is there anything wrong with using lift blocks over Add-A-Leafs? 

For those who don't want to alter the quality of ride, a lift block is a good inexpensive option. Unlike AALs, which can raise the spring rate of the factory packs resulting in a stiffer ride, blocks just lower the rear axle creating a new perch for the center bolt head to engage into. A lot of people will hear negative reviews on blocks and for this reason avoid them altogether. Though we are not big fans of taller blocks (over 2") as they can contribute to things like axle wrap or premature wear, smaller blocks typically have minimal to no impact on the integrity of the factory packs. 

Should I go with an add-a-leaf or a full leaf pack replacement?

This comes down to what you use your truck for (hauling, towing, consistent weight) and what your budget looks like. AAL's in general will provide you with some increased capabilities; however, they are very much dependent on the condition of the factory leaf packs. That said if your factory packs are lacking in any way an AAL might not be the best option.

Aftermarket leaf packs, depending on which one, tend to be more capable over the factory packs. In some cases aftermarket leaf packs can be designed to suite your exact needs. Though this is perhaps the best possible option for those looking for that specific lift height and/or towing capability, leafs packs can be quite expensive. 

Do I need longer u-bolts with the single leaf add-a-leaf?

When using the single leaf AAL (our Deaver AAL), longer u-bolts are not required.  As long as the factory ones have not been modified there should be enough thread on the factory ubolts to accept the addition of the AAL. Keep in mind most people do not like to re-torque ubolts and so if you want a new set we do have our HS ubolts available in our rear hardware section.