Well, if it's doing it only in 2wd, that would suggest the rear shaft joints, if it is indeed the problem. The one that is bad on mine is on the front shaft. I have 135,000 miles on mine and they are all original, so, considering the history of bad U-joints on R51's, I'm going to replace all four joints. I have steel driveshaft yokes on my 06 LE, so for the front shaft I'm getting a pair of Moog #280 super-strength, greasable u-joints for the front shaft. For the rear shaft, I'm getting a pair of Dana/Spicer #513301X heavy-duty, premium greasable u-joints (also known as the Spicer Life Series), which are a little more money, but look like they are built a bit tougher than the Moog #270 joints, which was my second choice. I bought them all from Rockauto.com and they cost $81 with shipping. If you go with the Moog #270 joints for the rear shaft instead of the Dana units, the total cost is around $25 cheaper, if I remember correctly.
Moog 280:
http://www.rockauto.com/catalog/moreinf ... cc=1433013
Dana/Spicer 5-1330-1X:
http://www.rockauto.com/catalog/moreinf ... cc=1433013