Not trying to be rude, but if you plan on off-roading your truck I would avoid using spacers. If they even make them for these trucks. Depending on the type of spacer you're using, a lot of damage to your suspension can occur...whether it be a reduction of up travel or your shock being the limiting factor on down travel. Spacer lifts are cheap for a reason. I personally can't see putting a cheap lift on a 50K truck, but to each their own.
On the other hand, if the mall speed bump is the toughest obstacle you will encounter and/or additional tire clearance is your end goal, it's a cheap alternative.
People seem to confuse strut/coil spacers with lift blocks. A lift block increases leverage on the leaves, causing axle wrap etc. A strut spacer has the same effect as moving the mounting point down. The only kit available for the air ride (BDS) runs spacers front and rear. It has all the other things too correct suspension geometry, but those are not needed at 1-2" of lift. I've wheeled an 09 wrangler through the rubicon on a 3" spacer lift and have never had a single problem.
You should have no problem with two inches or so of spacer lift. Much more and you may need to drop the front diff. Have not seen the bds kit, i would think they have to use drop brackets in the front to get 4 inches. It is no different than lifting a coil spring truck. Airbags are not magic, just pretend they are coils.
I'm well aware of the differences between coil spacers and lift blocks, and neither are ideal. All I was trying to point out was that there are much better alternatives to lifting a vehicle than cheap spacers or blocks. Drop bracket lifts don't, in themselves, provide more ground clearance. While they "lift" the vehicle, ground clearance is achieved by the larger tire size you can run with them. And, yes, the air bags replace the coils in these trucks, so, no they're not magic. I'm not just talking out of my a**, I have experience with these things as I've owned modified 4Runners and Tacomas. When it comes to others trucks and the decisions they make, I could care less. Some people probably wouldn't approve of some of the things I've done to past rigs, but to each their own. Just trying to provide some info for people who may be getting into the off-road scene for the first time. I'll leave this here in case it can be informative... The Little-Known Truth About Lift Kits and Ground Clearance