rear seal is leaking
crank shaft seal replacement on 2000 Chevrolet Astro
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..
agree with good guy or ck www.alldatadiy.com
you will have take the tranny out to change the crank seal,if it is the rear main seal then you will have to drop the oil pan providing its a two piece seal