A 4.0L inline-6 from most years of the Jeep Grand Cherokee (and other Jeep models like the Cherokee XJ) will generally fit, but there can be minor differences. The best fit will be from a 1999-2004 Grand Cherokee, as these are the most directly compatible. However, even those years might have minor variations in accessories (like the alternator bracket).
Engines from earlier years (pre-1999) might also fit, but more adaptation might be required due to changes in emissions systems, sensors, and other components. Going significantly earlier is generally not recommended due to increased challenges.
In short: While a 4.0L from many years will physically fit, a 1999-2004 Grand Cherokee 4.0L is the safest and easiest swap. Always double-check specific engine codes and compare parts diagrams to ensure compatibility before committing to a swap.
