the truck has a 351cid engine and at highway speed oil leakes out from back. wont leak at idle.

Could be your rear main seal on your motor. It won't leak at idle w/ low oil pump pressures but when you up the rpms it will. Try having someone rev it some and keep it at a steady 4k rpm and look under it and see if it leaks.

It could also be the oil pan or pan gasket.Ford oil pans are known to rust out a lot and the dealers stock them.