When changing my radiator (94 Chevy k1500 4x4 ext) do I need to plug the transmission hoses after disconnecting them?
You don't need to plug them while they are disconnected, unless the vehicle is going to sit for a while. You obviously don't want dirt getting into them, but if this is a quick in/out to replace the radiator, it's not going to be an issue. You may see a little drainage of transmission fluid from the lines, so plan accordingly. If they are flexible lines you can suspend them so they won't leak onto whatever surface. If you are going to leave the vehicle for a while, you'd want to put something over the ends of the lines just to prevent foreign matter from entering. A small plastic bag with rubber bands would work just peachie. If you have some old nitrile gloves, you can cut a couple of fingers off of them and do the same with the rubber bands. They are about the right size and fit on there nicely.