1999 ranger trans problem

i have a 1999 ranger with a 3.0 engine with auto trans, 140000 miles,the trans worked fine till i put a rebuilt motor in the truck,i now have 2000 miles on the new engine, the trans works and shifts fine till you accerate hard into the passing gear, it jerks and shifts erradic ,seems like it shifts into the passing gear, slips then shifts again, i changed the trans filter and the fluid is at the right level, i checked all the wire connections and they seem to be all hooked up,i could have missed something, it drives and shifts just great, what controls the passing gear shifting??:frown: thanks in advance skip