BEFORE MY 99 TJ WARMS UP TO OPERATING TEMP, AND WHILE BACKING UP, IT FEELS LIKE THE CLUTCH IS SLIPPING.  IT SORT OF VIBRATES LIKE THE CLUTCH IS RAPIDLY ENGAGING/DISENGAGING.  AFTER IT WARMS UP, IT'S SMOOTH.  ANY IDEA WHAT'S WRONG.  IT'S BEEN DOING THIS FOR THREE YEARS. THANKS  

twyatt:
BEFORE MY 99 TJ WARMS UP TO OPERATING TEMP, AND WHILE BACKING UP, IT FEELS LIKE THE CLUTCH IS SLIPPING.  IT SORT OF VIBRATES LIKE THE CLUTCH IS RAPIDLY ENGAGING/DISENGAGING.  AFTER IT WARMS UP, IT'S SMOOTH.  ANY IDEA WHAT'S WRONG.  IT'S BEEN DOING THIS FOR THREE YEARS. THANKS

You have what is called clutch chatter. It is caused by a slightly warped or hot spotted pressure plate or flywheel usually. As it heats up the warpage can change a bit along with frictional properties of disc and it can go away. Reverse is a bit taller than first and you tend to "play it" more so it places more strain of clutch than going forward.