Cherokee 4.0 L st6 automatic runs ok till it starts up hill then it starts shaking until I press on gas and transmission downshifts, vibration then smooths out.

What is going on?

I put a quart of Marvel Mystery oil in it a few days ago to quiet down a clacking sound in the cylinder head. This seems to have worked, but now there is is vibration under load.

I would appreciate some suggestions,
Thanks