after engine swap , engine begins to run very rough as soon as it enters closed loop what would cause this ?

had a engine from a 1992 toyota 2wd which ran fine ,put into my truck , a 1994 4x4 ext cab . my engine ran fine also but had overheating issues along with engine noise issues , now that the engine from the 92 is in the 94 it starts and runs good from cold but as soon as it enters into closed loop begins running terrible the shop doing the work for me cannot figure it out ,, would you have any answers for us ? or do I now have a useless truck