1991 Chevy Silverado coolant in oil

It is either a head gasket or else the block or a cylinder head is cracked. You can usually find the problem area by checking the amount of compression in each cylinder. The one (or more) that shows a lower reading will be the one where the leak is occuring. You may also see rust on the end of the spark plug. It is not hard to do this check if you have a little experience and the right tools. If you don't have both, I would advise having a pro do the check for you.