Just where is the leak at? If it is above the starter only on that side, you are probably looking at the head gasket leaking, not a rear main. I haven't replaced a rear main in a 5.4 ever. Although it is a possibility. Just really take a look at the back of the pass cylinder head. The oil pressure feed to the head is located back there and in 99, Ford changed the gasket and it had a problem, so they switched back. I know it is a 2000, but I bet the engine was made in 99. The rear main bearing is internal to the engine and if it goes bad, you will have excessive lateral movement on the crankshaft.
Last edited by bigbronc; 02-23-2009 at 11:10 AM.