Jeep Engine replacement

We have two Jeeps that we need to replace the 4.0 engines on. A 1996 2 wheel drive and a 1998 4 wheel drive. We have found a 1994 2 wheel drive engine that we are planning on putting into the 1996. We have also found a 4 wheel drive replacement engine for the 1998 but it is out of either a 1994 or a 1996 (I can't remember for sure). My question is this. Will these engines be compatible with our Jeeps? We have heard that an engine for the 2WD has to come from another 2WD and also the same with the 4WD but our concern is the age difference in the replacement engines. Miles on each used engine is very close to the miles currently on our Jeeps. Thanks you so much for your time in considering my question.