If the 4.0 has been taken care of 100,000 miles is nothing. My 2006 had 90,000 on it when I put the Xcharger on it. When I sold it last June it had 203,000 on it and was still running strong and making 12 second passes at the track. The new owner has it up to 211,000 and said that he had a leak down and compression test done and it is still in good shape.
And used 4.0's are cheap - you can get one for $400, which is why most don't bother to rebuild when the break one, just replace it.