Sorry but whatever they guy gave you is not caster. If you had caster of 1.5 degrees your Jeep would be very difficult to drive.
Assuming you have the PR unlimited, just adjust your control arms so the pinion is in alignment with the front driveshaft. That will eliminate vibrations. Depending upon your lift, your resulting caster will be anywhere in the 4-7 degrees range, which is fine. I have a 4.5” lift and ended up with 5 degrees of caster with a PR44 unlimited.
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