The usual path is a blown head gasket or cracked block. If it was running normally when you changed the pump then I suspect it happened when you did that. I do not remember if this applies to the mighty ram but on my Dakota the front of the oil pan is exposed or only covered with a bit of rubber when you have the TC cover off to do the WP. If that is also true of the might rams then you could have let coolant drip down into the front of the oil pan. Wish I could be more helpful. Some other guys on here know a lot more than I do they'll speak up when they login.