If not done already, remove the engine cover and make SURE it's not the oil sending unit or the distributor o-ring seal that's leaking!! Either of these can leak oil down the rear of the engine and appear to be coming from the rear crankshaft seal. This 4.3 engine has a very reliable, one piece, round crankshaft seal, which requires transmission removal to access. "IF" it is indeed this seal!!! More common for oil to leak elsewhere however rear seal is possible..