Are the engines swappable?

With a 4 year difference between the engines, the ENGINES should be swappable, but the WIRING as well as the engine accessories, gauges, and sensors LIKELY are not all EXACTLY the same on both vehices or engines. Plus you didn't say if both are engines hooked up to the same TYPE of transmission (MANUAL OR AUTO), or MODEL of transmission. The ECU for an AT vehicle is often a different PART (or is calibrated differently from teh factory) than when used on a manual trans vehicle. The factory accessory packages may be such that the ECU's are caibrated VERY differently, so even if one happens to work on the other engine, it may be much less efficient than using the correctly calibrated one. For example an engine in a vehicle with a heavy duty factory tow package may need to have a bunch of different programmed-in computer controlled settings at vatrious rpm s than one with no tow package and a higher numerical gear ratio. Also, emission controls, (O2 sensors, catalytic convertors, etc., etc.) and their associated settings can change from year to year and the ECU takes all the sensor inputs into account when adjusting the injectors and engine timing for various conditions. In short, when switching to a different model year engine you may have to swap over the ECU along with the engine and possibly also swap some sensors that are NOT mounted on the engine, but are different from one engine and vehicle to the other. Undfortunately, the only way to be sure if things such as sensors and ECU's ARE the same (even if they LOOK the same) is to look up the parts for both vehicles.

98-2000 engines are compatible. Among other changes to the engine the motor mounts are different in the 2003.