In my opinion the deadlift is the most difficult to do of the big three lifts, 90% of people dead lifting in the gym use terrible form. To get deadlifts right you really have to keep your back straight and focus on the lift. For the grip problem I would say buy a pair of straps but don't bother using them until the set where you feel your grip slipping.
All this being said your deadlift is unrealistically low compared to your bench and squat. Therefore there must be an injury or a serious form problem. If it really was an injury I would think that the squat would aggravate the same spot since they work out many of the same muscles and both put pressure on your lower back. As I said in my comment I'm going to assume that your problem is actually that you are not going deep enough to make the squat or bench difficult. When you do a full ATG squat you definitely feel it in your back after, I can't imagine that this wouldn't aggravate the same back injury as the deadlift.
If you don't believe that your bench and squat form are the issue I suggest you see a doctor.
Front squats are more helpful in increasing you deadlift because of where the load is balanced you may want to incorporate them into your routine. If they also hurt I would suggest you see a doctor. If they don't it's possible that your weak grip causes you to compromise form. Chalk may also help with your grip. I Also saw you said that the sumo didn't hurt is much, generally sumo dead's are easier on peoples backs but most people lift less with them for the sake of your back it might be a good idea to stick with them. Also since your squats are low bar a squat higher than your deadlift wouldn't be to ridiculous but personally my bench is about 62% of my deadlift and I do not consider myself a good deadlifter