91 Cherokee Bucking under smallest load

Recently purchased this vehicle so have no history. It is a '91 Cherokee Limited with the 4.0L High Output inline 6. It has a Select-Trac system that properly functions. Have shifted through range and each range functions. The automatic transmission shifts properly in and out of the gear ranges and overdrive seems to function acceptably. Everything seems normal ON FLAT PAVEMENT. It cruises along nicely.The fun begins when driving up light inclines at any speed. I mention light inclines here because it doesnt take much of a load factor to start the vehicle into a bucking rough ride. It is like a chattering and the vehicle is definately at war with itself. It gets worse on steeper highway inclines. This happens in the 2 wheel drive mode as well. Transmission fluid level is "full" and all 4 tires are the same size and make, although very low on tread, they all have the same wear factor and are wearing evently across the width of the tire. It has the stock 15" factory alloys. The engine runs strong and smooth at idle & througout the power range. It doesn't misfire.
Also, The are not any loud noises associated with this problem.

Any Ideas?

For most vehicles I have owned this has been a clogged fuel filter.