hammy we just pulled/installed two 1998 4.3's this last week. The one with 524K miles rear main did NOT leak. The one with 119K miles did not leak at the rear main and the flywheel on each were dry.
We did go to GM and get a new rear main seal for $40 for the 119K mile engine since they better built then FelPro after market rear main seals from what I could learn while it was on the engine stand.
It was a good running engine when pulled from a 4WD auto 1998 Blazer but it was wet from top to bottom with engine oil and had OEM intake manifold gaskets. So when on the engine stand we stripped it down to a long block and came back with all new gaskets before installing it into the S10.
At 119K miles the OEM intake manifold gaskets looked like wet cardboard and were sure to fail sooner than later.
Short of a factory defect you do NOT have a rear main seal issue as others have noted. Even then only a 5-speed would make me concerned. Lost a Datsun clutch many moons ago due to a leaking rear main engine seal. Well the clutch was not worn out but sure was slick.
We were putting the 119K in front of a 5-speed. Manual transmissions do push the crankshaft forward a bit each time you let out on the clutch so they may get a bit more stress but the OEM rear seal was fine after 524K interstate miles chasing new mobile homes across the country. The flywheel had a grove cut in it by the clutch band.