For any 4wd Titan, 1.5" is the biggest leveling kit you can put on, unless you add some aftermarket Upper control arms (UCA's). If you add upper control arms you can fit any where from 2.5-3" on it, depending on what upper control arms you put on. Anything over 1.5" will cause the OEM upper control arms to hit the coil bucket, other wise know as CBC (coil bucket contact). I will eventually cause your UCA's to fail.
Any idea... would the Bilstein adjustable struts give me additional 1.5" I'm looking for? Or will they only take me to where I'm at right now? (I know the pro4x is higher than a 2wd), not sure if I go with those if they will give me the extra leveling height I'm looking for. I see in your signature you have the 1.5 spacer AND the BS5100's.
Yes the Bilsteins will give you the height, either 1" or 2" depending on the setting. From what I understand and many other have experienced though, the top setting is a pretty firm ride, and you run into the same problem, anything over 1.5" your going to need UCA's. I have been running the Bilsteins on the middle setting and a 1.5" spacer for about 6 years now, but I also have Dirt King UCA's, that is why I can run the additional height.