OT- Grand Cherokee Problem

The car:

1997 Grand Cherokee with 5.2V8 and 4 speed auto. 213,000 miles.

The car runs just fine, but if I drive up any kind of incline at over 70-75mph it feels like I am driving on rumble strips when i have my foot on the gas. If i take my foot off the gas it goes away immedaitly, put my foot back on the gas and it comes right back. It only happens while going uphill at high speeds.

Anyone know what it could be? The car is on its original transmission etc etc. The car has really never had any non-routine service.

Mike