motor swap

You can but. The 98 model 4.0 explorer came with an over head valve engine. The 2001 may have a SOHC engine. The PCM hook-up for SOHC engine does not match that of a OHV engine. And you will probably have several sensors on the SOHC engine the 98 doesn't have and no PCM plugin for. My 99 finally required a new OHV engine. The labor to pull out the old and put in the new was the same regardless of engine. A new reman Motorcraft engine cost about 3000 bucks, it was a drop-in and guaranteed by Ford with some ridiculously high mileage and years of service. Compare the price of a new Ford engine vs something you find around and make a decision.