I am writing this article for the simple fact of this question is hardly ever that i could find directly answered. i may have missed it somewhere but i looked for awhile back when i had this problem.
On the back of your NP231 tcase there is a machined fit ring it has a inner and a outer ring on it with rubber bonding the two together. after awhile either that rubber rots or with a lift the vibes have their way with this rubber and all the sudden THUMP THUMP THUMP comes from under your poor little jeep.
There are articles on here saying you dont need to run with the ring. FALSE. take this from someone who has had the worst outcome from this experience. that ring acts as sort of a Harmonic Balancer for your driveline. it helps dampen the vibes coming from your driveline. without that part what will happen is you will end up having worse vibes. and if your u joint hold up somehow like mine did you end up starting to bust teeth off your ring gear. also all the vibes it causes actually bent my output shaft by .100" causing me to really get the shaft of this deal.
If that ring comes off use a gear puller to remove the remaining inner ring and call your local chrysler/jeep dealer and they should be able to fix you up. i picked mine up for 145 bucks. another reason to be fooled by this part is i had only found one website that sold it and that was of course mopar. but not having any pictures you arent quite sure on what your exactly buying.
Finally to stop all this headache just call up Tom Woods and have him ship you a new driveshaft and AA SYE kit to your door.