I don' t know the bolt or thread size, but you can get them from Mopar if you PM them. Benny's pretty responsive.
Hopefully someone from the forum will know the size.
There are 4 M8x1.25 bolts (Grade 10.9), on the inside and outside of the frame rails. The other 8 bolts that hold it on (attached by the tow hooks) I'm not sure about. I think they're larger than the others, probably M12.
Edit: I just realized the M8's I was talking about were made for the winch plate mount. The ones for the bumper came off with a 16mm socket, which points toward an M10 bolt. 18mm socket for the other 8, which suggests an M12 bolt. There's no exact measurement, but it's a good indicator.
I know this is an old thread but just in case this helps anyone out while searching... I just got done mounting an aftermarket winch plate and the bolts used for the frame rails where the winch plate mounts to are M10x1.5 grade 10.9.