I dont have dhoes or UCA's but after reading many threads i think i understand. one of the main reasons people are getting the uca is because on some trucks after alignment the castor is off and not to factory specs. so with the new uca you have more adjustment in castor and can get it back to factory specs. also, i think it may give you slightly more travel in suspension. oh and they look cool. it is also possible you may not need them at all, so what i would do is: install the dhoes, get an alignment. if it comes back not within factory specs, then get the uca. if it comes within factory specs, then you are ok and dont need to spend any more money.
If you add more than 2" of lift your starting to shorten your travel.
Otherwise stated you can still align the truck properly with 2" of lift, at three you may be able to get the alignment done but your at the end of the range... So for 3" of lift it's better to have the UCA.