It could be a bad rear u-joint. It can also be an idle that is set too high allowing for a severe backlash when the gears are changed. AS was stated above, parking on an incline with an automatic requires the proper use of the emergency brake in order to take the stress off of the drivetrain.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...