Question does it do the same when not running?. If it doe's not most likley problem is a cracked fluid line from the opening and closing of the rear hatch, it's not a rare issue with the cherokees. Best why to find out is start the truck sit in the back and have someone open the hatch and listen for sound of a pump running to move the washer fluid. I would still check the fluid lines even if you don't hear anything. If it's not the hoses then posibly a seal on the hatch or a clog in the fluid lines.
Thats my best answer with the current information best of luck.
Oct 8th, 2013
Then i would bet it's either a line came off the housing plug or a crack in the line itself .