You gotta take all the opinions on the forums with a grain of salt, the bilsteins ride just fine. It is not the brand of shocks that is the problem, it is putting more preload on the spring (i.e. to get more lift). The compressed spring is what stiffens the ride, but even the bilsteins on the 2" setting I have never thought where rough or crappy to ride on.
The rancho quick lifts may not work with the rize strut spacers since the adjustment nob is on the bottom of the shock shaft and the rize lower spacer clamps around most of the lower end of the shock. Plus preloading the spring to 2" and then turning the dampening all the way down might have adverse affects on the handling with such a large lift and wheel/tire combo. I have never heard/seen the rancho/rize setup being run, so who knows really, but judging from what I have seen/done I'd stick with beilstein or at least try to find someone with the rize/rancho setup(?) Its hard to say on that setup.
you could use the 2.5, I have known many people who have, but I wouldn't recommend it either. It is not so much that it would tear the CV boots (I don't get why shops always say that..), but they would be constantly rubbing 'ribs' if you will on the boot. So, instead of being spread apart with room to breath/flex, now the angle of the suspension makes them always touching on the bottom of the inside knuckle, and always touching on the top of the outside knuckle of the CV. They would probably tear up pretty easily with heavy offroading. I'd hold off till you could swing the coilovers or the bilstein setup (more lift and better suspension capability/performance), I think you would be much happier with either of those setups vs. just getting more lift now with the spacer.. Any of the three ways will work though, it just comes down to what you want to get out of it and how much you want to spend.
the ride will change, and if you are 'ride sensitive' and need a factory feeling, then yes it will change it. BUT the change just makes it feel more like a 'truck' it still has plenty of cush to the ride, but you know you are in a lifted truck and it rides like a lifted truck should. Not bad, but not butter smooth like factory. I backed out of the shop on bilsteins set at 2" and had zero complaints. in fact they rode better than a set of off the shelf kings for an 8" lift cranked up 2" (WAY better). Now with custom valving and internal guts on the kings they beat up the bilsteins.
The bilsteins at 2" will be just fine, hell everyone is running similar setups with 6", 4", etc lifts... They probably just advise not to for liability reasons should something go wrong and they don't want everyone calling and bitching when a truck rides like it should suddenly.