I've read more threads on this than you can imagine, and have found no definitive answer. Many say set your caster around 6* for 3.5"-4" lift. Other threads talk about 6*+ of caster will destroy your brushings and T-case and should keep it closer to stock.
SO HERE'S MY QUESTION:
My caster is at 6 and 6.2, jeep drives great and I do not feel hardly any vibes (haven't been on highway with high speeds yet). Am I safe with my caster there or should I dial it back some. Don't feel like buy another T-case.

My info
I have 3.5 lift with LCAs and stock axle and driveshaft.

 

You haven't found any definitive answers because there aren't any. It's case by case with each specific build, and what compromise between caster and pinion you feel comfortable with.

'Hardly any vibes' sounds like the pinion is tilted down a bit to far, and you are, in fact, feeling driveline vibrations? And what you want is for someone to tell you it's ok to have those driveline vibes, right? It's your jeep. If you want to drive with vibrations, drive with vibrations. If it never causes a failure, good for you. If it does, you'll know for next time.

(The tcase explosions seemed to be a combination of factors, not solely attributed to caster. It is just one of the things to factor into your decision)

 

Caster should be 4.2-4.5 degrees which is the factory specs, you can air on the heavier side and we have seen JK's with upward of 5.5-6 degrees of caster and still not have any vibrations. Obviously the more angle on the cv boots the more prone they are going to be to failure but it all depends on how you use your rig.

RK

 

Thanks for the help, I'm going to back in down to 5ish to be safe

 

I have caster angles of 3.7 driver side and 3.9 passenger side will I have vibration issues with a cardan front drive shaft? 2.5" stock mod lift.

 

I have a 4" Rancho lift and they recommend 4.6*. It's spelled out in their installation instructions. I do not have LCAs but because my kit included geometry correction brackets designed for my lift, my castor is right on spec.

If you know who made your lift maybe you could contact them and get the design caster directly from them. We hope that everyone who manufactures a lift kit goes thru extensive design and testing to make sure it will perform optimally.

The axle/pinion angle in the second picture below is worse case at maximum axle droop. At normal ride of 5" over stock for front that angle is less than 2*.

 

A little insight. Caster and pinion angle are zero sum. As caster increase, pinion decreases. The stock axle has 6 degrees of separation. For example, when caster is 6*, pinion is 0*. Caster at 5*, pinion at 1*. Etc. With caster above 6*, your pinion will be pointing down. Not a great idea.

Next, think about axle rotation. If your upper arms are shorter than the lowers, what happens when your axle droops? Caster increases and pinion decreases. The uppers follow a tighter arc. Think geometry.

Long arms reduce the effect of this issue, which is why you see those with a lot of lift or suspension travel going with a long arm lift.