Bushings do get warm when you drive due to friction. Not very warm but enough to make a little difference. I have never ever ever ever had a death wobble on my jeep when its cold, infact. Only ever happened on warm days after driving for about half an hour.
Could very well be an out of balance tire. that only has an effect when worn bushings are warm. Wheel weights fall off, tires wear unevenly, etc. Could be belt seperation at your wear level. Have you closely looked at your tires?