It's the crank sensor!
Join Date: Nov 2007
Location: Central Minnesota
$1300 - $2000 depending on a lot of variables. Be sure you get at least 2, better to get 3 quotes before making any decision.
And as mentioned, consider a plug and play salvage yard engine. To each their own on these decisions.
A very minor knock may not necessitate a rebuild. The knock could be a bit of piston slap or valve noise; neither of these uncommon with the 4.0
What I recommend for these is to DRIVE IT, and keep an ear to it. If the noise gets no worse, keep driving it. I have seen many, many 4.0 engines run for tens of thousands of miles with a bit of a knock.
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!