On the Explorer, there are 2 main culprits for antifreeze in the oil, head gaskets and lower intake manifold gaskets. Both of which require some level of engine disassembly. You need to stop driving it now and get it fixed. If you don't you may ruin the motor completely. Besides the overheating issue, antifreeze doesn't have the same lubricity as oil, which may cause premature wear of components that rely upon the oil for protection.
First you need to figure out where the problem is. Since you already have antifreeze in the oil, you don't need to do a radiator pressure test because that will only tell you that antifreeze is leaking. You need to have a compression test done to determine if it's leaking into the cylinders. If it is, then that's most likely a bad head gasket or a warped head. If it isn't, then it's likely to be the lower intake gasket.
One thing you might want to try is re-torquing the lower intake manifold bolts. I have heard of this fixing antifreeze leaks in the past. However, if the gasket is damaged, or it's a head gasket, it won't fix the problem.
