I have a new Duramax and want to get rid of the 245s and get something along the lines of a 285 or 295.

Most of the 285s or 295s are a load range D.

The weight rating is actually higher on the bigger tires although they are a lighter duty tire.

Is a load range D with a higher capacity ok or should I try to find some Es?

Unless your hauling heavy loads the Load Range D is sufficient.

BFG makes a 285/70/16 E rated if you want a tire that will last thats it.
Other D rated tires will wear sooner.

Not that I'm going to beat this thing to death but if your not hauling heavy, LRE will not last any better than LRD.IMO

I got 265's in a D rating, they have a higher weight rating than the stockers and have worked great for me.

BFG also makes a 285/75/16 All Terrain with an E rating. The E rated tires are more stable with loads. The sidewalls do not flex as much. The largers D's will carry the weight due to the additional air volume, but at a lower pressure causing more sidewall flex. If you do not tow much, or at close to max weight capacity, the D's are fine. I would get the E rated if you can find a tire you like. Costco here in Texas will not even mount a D rated tire on a truck that came with E's from the factory. They will not even mount one upsize either. I am guessing liabilty raised it's ugly head somewhere.

On your door build sticker, there will be a GVWR rating. That tells you how heavy the rear of the truck will be. Divide that by two and compare that to the weight capacity listed on the tire sidewall. The two tires should add up higher than GVWR. You never want them to be lower. The Load Range Letter is less important than actual capacity. Most 245 E range are 3000+- and most 265 E range are 3400+-, hence 265's on 3500SRWs. The only other side effect of dropping a Load Range from E to D is that while the ride may soften up, the handling can turn downright sloppy.

BTW, some stock 265's (Discover ATR's) will rub the bumper on occasion on my 3500SRW trucks. I can only imagine a bigger tire may rub more. Plus, have your tire shop check if the tire you want will fit a 7.5" rim. If you go too wide, inflation has to be carefully monitored and adjusted for tire wear, as the tire will tend to bulge at the center of the tread (they'll look like you're doing four wheel burnout's by 10k miles or less).