I have RC fixed arms which are 16" long and purchased adjustable UCA to fix pinion/caster angle.

I just got RE 4.5" which is about 5-5.5" lift can't I just use the UCA to fix pinion angle why the need for 16 5/8th-17" LCAs if I can adjust with UCA?

Is it because with the lift the tires aren't centred properly?

Edit: so stock mounts can be shimmed Up To 3/4"?

 

Kind of unclear as to what you are asking, but, if you are wondering why you are supposed to get longer LCAs with a bigger lift it's because, as the Jeep goes up the control arms pivot the axle downward and in a circular arc back, if you don't get longer arms the axle won't be centered in your wheel well.

 

I had some issues with suspension geometry before I went to adjustable control arms. For me I had a slight shake, sometimes even death wobble, with the stock arms. I had a lot of other issues going on with my geometry but overall the longer control arms did seem to solve some of my issues. Hope this helps!

 

use your adjustable uppers to point the pinion at the T-Case and hope for the best. The Jeep will let you know if you need longer LCA's via vibes, death wobble, creaking noises, and such.

 

One more vote for longer LCAs to center wheel in wheel well. Other than that, I don't see where it makes a difference. As far as adjustment of lower arms, I was able to shim mine 5/8" IIRC before the nut was flush with the end of the threaded shaft the shims are on. I wouldn't feel comfortable shimming mine 3/4".

In your position, I would shim the LCAs as far as possible while maintaining good thread engagement on the adjustment nuts. Then I would set the caster with the UCAs and drive it. See what happens. Remember caster and pinion angle are directly related, it is a compromise to get good caster while not screwing the pinion angle up too bad.