I'd recommend not putting any more than about 5lb of force on the shifter. Sounds like you may be stopped and in neutral; if so, have someone push the jeep from behind while you're gently nudging the shifter into a gear. If you can't do that, you could lift the front and rear tires off the ground and spin the driveshaft(s) by hand a little. If you end up breaking the cable ends, Advance Adapters makes a nice replacement with little steel heim joints instead of the cheap plastic, but it's still no excuse for moving the lever gently. My original cable never failed me when out and about, but when I disconnected it for another project (11 year-old jeep), the plastic ends basically disintegrated in my hands. Good luck!