I just went through this exact situation. Leaking upper seal of steering gear box. I tried Lucas and it may slow it but in the end the seal needs replaced. You can buy a new seal kit but to replace it you must remove the entire box, and since you have to remove it anyway you are better off replacing the box. Often times when the upper seal is leaking the bearings in the box will soon follow. You can get a rebuilt box: Lares part #1353 from rockauto.com for 58 + 35 core charge. It is a pretty simple install, start spraying down your pitman arm link a week before and rent a good pitman pulled from your local auto parts store (that way if it breaks you just take it back in, they have a warranty on it). Stu has a great write up on the replacement here
You may also consider looking into and reading about the durango steering box upgrade, here is a guide on that
Whatever you choose, good luck and feel free to pm me if you have further questions.