Yeah, freeze plugs are a very good possibility, there are two of them on the backside of the block. Unfortunately the only way to get to them is to pull the tranny or engine, since they are behind the flywheel. There is no way to even see them otherwise.... probably the most expensive $3 part in the whole truck.
While you're getting it done, if you are still on the original clutch and if your mileage is over 50k or so, might as well put a new one in if you can afford it. Save a lot of labor cost down the road, since they have to take the old one off anyways.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.