work on grip strength at the end of your arm day. id say its probably because youre using your grip alot more on back day because its when you use the most weight. ie: up right rows, seated rows, deadlifts, bent over rows
check out Ironmind's Eagle Loops. i've been using them for a few weeks on chinups and timed holds and i have already noticed a bit of an increase in my grip strength from them...but im also doing work with a gripper and doing some strap holds every once in a while.
if any of that stuff interests you, they also make straps which you can probably get anywhere, but if you were to order you might as well throw those in too.
note: im not an advertisement, i just had the site open in another firefox tab
This could be from a few things. You don't need to grip the shit of the bar or whatever you are using. Just grip it enough to hold it. Obviously if you need to use max grip to hold the bar that isn't an option so you want your hands to be pretty strong. You also want to concentrate on not pulling with your hands, but more with your elbows. A little bit of concentration on a certain body part goes a long way when you're lifting. Like if you're doing bent over rows focus on pulling your elbows all the way back instead of your hands pulling the weight. This will help your grip and also put a lot more focus on your back. I also like to use an overhand thumbless grip for a lot of pulling exercises. Doing this makes a big difference. This is much easier with straps but if your hands are strong you should be ok. Just don't sacrifice your back workout by not using straps if you need them.