By increasing Caster angle to a higher positive value you are leaning the top steering axis that passes through the steering knuckle towards the back of your JK. If you look at your axle from the driver's side it would rotate clockwise.

By doing so you are rotating your pinion downwards which increases the pinion angle. High pinion differentials help mitigate this problem to some extent.

 

I didn't have any vibration until I had the new gears and Coast shafts installed.
Now at 50-55 mph I get a vib. It goes away at 60 and up.
I got under it and took a look Saturday.
The rear pinion looks to be almost perfectly in line with the drive shaft.
The front was off quite a bit.
I had recently had the front end aligned at the jeep dealership and the castor was set to factory specs.
I decided to lengthen the uppers to try and take some of the angle out.
I lengthened them 4 full turns and bolted them back up.
It helped with the angle, but it made the steering a little bit "twichy".

The change didn't help the vibration though.
I may change it back the way it was.