One of the most common issues with the 4.0 engine in the later model Wrangler TJs is the OPDA (oil pump drive assembly). The factory OPDA is designed with two oil impregnated bronze bushings separated by an oil seal. The upper bushing is cut off from lubrication by that seal, resulting in premature failure. Over time, radial movement develops in the shaft. Eventually, the complete assembly and camshaft will fail.
Jeep / Chrysler is well aware of the OPDA issue with these cars as they issued the E05 recall for some of the 2005 model 4.0 engines due to premature oil pump drive gear wear. This recall involved engines that were built between 10/20/2004 and 02/01/2005. However, don't be fooled by the recall dates as there are many owners of 2006 models (and even the years prior to 2005) that have experienced OPDA issues, including complete failure.
A worn oil pump drive assembly will show symptoms, such as a check engine light or a squeaking noise coming from the oil pump drive assembly itself (some people describe the noise as a loud screeching sound or something similar to a laughing barrel of monkeys).
The fact of the matter however is this is no laughing matter. Ignoring a bad OPDA can and will result in catastrophic engine failure which will put you in the hole for thousands of dollars. On the contrary, a replacement OPDA is only a bit more than $100, and it's a very easy thing to change out as well.
If you recently purchased a used Wrangler TJ one of the first things you should think about doing is to change out the OPDA with the redesigned Crown Automotive OPDA, regardless of what year your Wrangler TJ is.
The reengineered Crown OPDA moves the seal above the upper bushings, allowing oil to lubricate it. An oil channel was also added to the shaft to increase overall lubrication to the area.
You can purchase the Crown OPDA here.
The video below perfectly demonstrates step-by-step how to change out the OPDA.