I was looking into 37's and noticed the weight of some like the Goodyear mtr's, Bf Goodrich km2's and Nitto Trail Grapplers 37's weighed less than my 35 toyo open country's that I have now. If I run a lighter 37 than my 35's will it cause axle problems? Basically what I'm asking: Is it tire weight or tire diameter that causes things to break?
Im also running the stock d30 in the front with an artec truss and c gussets and its held up great so far wheeling with the 35's

 

Both. I think the worst cause of breakage is spinning the tires when they begin to bounce. Usually the larger tire is heavier but when you get to these "E" rated tires that are for 1 ton trucks they are always heavier even in a smaller size so it's a toss up. I just try & use as little throttle as possible and I rarely climb in low range 1rst gear because it's so easy to spin the tires.

 

Both, but the size, IMO, would be harder on stuff than weight. The larger the lever arm the more force, same for tire size.