Try cleaning it and see what happens
Did u take it on a 10 mile drive?
If you didn't change the computer when you swapped engines there is very good chance it is the computer. Wasn't the replacement engine from a Explorer?
Cleaned the MAF sensor and its not throwing codes and not running rich. Check engine light ain't on. One more problem though....it hesistates at 3000 rpm... only when going up hil or on flat, not down hill....what could that problem be?