4.0 liter I6 is a very solid engine. They WILL make noise--especially tappets. Bottom end is strong unless oil changes were irregular.
Note that the 4.0 Liter from a Wrangler will not bolt into a Grand Cherokee. At least that's what the guy I bought the 4.0 Liter for my company Cherokee said. He had bought it for swapping into a Wrangler, but sold it to me when it didn't work in the Wrangler.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags