My 83 300 sd is leaking oil, it looks like it is leaking out of the front of the engine, right behind the main pulley. It is leaking a fair amount and I am worried. What could this be any ideas? I am wondering if it is a seal behind the pulley? Thanks for the help

Front main? Clean up the front of the engine and take a look.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Seal behind the main pulley? You bet. That would be the front crankshaft seal. It is prone to leaking once these old diesels get a few miles and some years running.
Good news is that one is not very expensive and not very hard to replace. Last on I put in took less than an hour start to finish.You will need to remove the radiator to get good access to it. Then pull the front pulley. The Haynes manual has a very good step by step procedure. Those six or eight allen headed screws inside the front pulley can be fun to get out. Will take an allen head socket style. Make sure to get the allen head tool shoved into the screws and to keep it very straight, or it will just round out the head.
Be sure to follow the procedure for the seal install carefully. It is best not to twist it around when tapping it in there.
Oh did I mention that the large bolt on the front of the crank will require 180- 200 ft lbs of torque to tighten back in there? Don't cheat, rent a high range torque wrench if you don't have one that goes to that range. You can really get into trouble if that bolt get loose.