Find an old tool used for setting spark plug gaps and the points on REAL cars. The real thin blades will help you get under the grip and loosen it up. Squirt some WD40 in there.

Now, go to a gas station or body shop. Get an air hose with a pointed nozzle. Stick it into the end hole on one grip and plug the other with some gum and then your finger. Blow the air in there. If the seal is adequate, doesn't have to be perfect, you'll see them start to slide off. If you can get someone to help, they can twist on one while the air is going in. You have the air working to expand the grip and force it off the end of the bar.