I've had a couple of threads on here the last couple of months seeking help on finding the pop/thud noise underneath my TJ. Had lots of help and some good ideas. I had to take it in anyway for AC and brake work so I had the local garage look for the noise. They thought they found it when they replaced the rear sway bar links. Nope! I finally took it to a Jeep dealer (cha-ching) 60 miles away. They found it.....the transfer case mount had two loose nuts.....they tightened them and noise gone.
Also, they investigated the occasional low oil pressure indication and alarm that I reported on yet another thread. I was 90% sure it was a faulty sending unit but the other 10% was bugging me. Sure enough, the oil pressure was great at all RPMs and the sending unit was bad. They replaced it.
Thanks to all that helped me find these problems! Fweaky suggested it (the pop/thud) might be the transfer case mount and I told the mechanic to check it when he was checking the catalytic converter. Many alerted me to the faulty oil pressure sending units and sure enough, it was. At least now I have the peace of mind....100%. Thanks all!