[Underpowered]

disc conversion would be pretty easy, bolt on actually. the ZJ (grand cherokee) came with the same rear axle, D35, and came with factory disc. all you would need is the whole brake setup off a Zj and it would bolt right up to your TJ.

but have you checked everything out on it? did you have the rotors and drums turned? or replaced? checked wheel cylinders? master cylinder possibly going south? vacuum to the booster clogged? many things would cause poor braking.