well, let's put it that way:
either your engine is plain tired and ready for a rebuild (did you do a compression check yet?) or you have too big tires for the gearing you have in there (as mentioned above, 33x15.5 is big for a 3.55 gearing). I run 32s with 3.20 gearing and it feels really boggy, especially at take off, but then again, I have the 2.71:1 reduction in my NP241 t-case which helps a lot getting those tires spinning in sand, if I have to. I will not guess what t-case you have in there, since you have such a variety of drive train components in your truck, but if you have say a 205 t-case you have only a 1.96 reduction, which just might not do the trick.
I'd say, you can basically try and upgrade your gearing, at least 3.90s or lower (numerical higher), or go to smaller tires. Probably the easiest. Or get a new engine/rebuild and if you just need to spin your tires in dirt with reduction, you can also look into a t-case upgrade or a klune-v box, which is a gearing reduction box.