My vehicle specifies a LT tire with a load range E rating.

Do I need to replace the current tires with the same E rating?

I am going to be towing my 10,000 pound trailer with approx 1,000 pound tongue weight but I don't really think I need that heavy of a load rating.

Am I off base or can I get by with a lighter D rating tire?

If the maker thought it was heavy enough for an E, why question them? UNless of course you:

A) have multiple advanced engineering degrees, like the guys who helped design your truck,

or:
B) think it's a conspiracy to make you spend more on tires.

You need to stick with load range E tires. Any reputable dealer will refuse to put any lower rating on there for liability reasons. Why risk it?
-Tire Industry Association Certified automotive tire technician
-6 years tire experience

You could probably get away with using D range tires based on the weight, but depending on your vehicle's year, you may have the tire pressure warning light on all of the time since the D tires can't be inflated as high as E tires.  Also, you run the risk of someone inflating the D tires up to truck's factory inflation specs.  That is a very bad thing to over inflate tires by that much.  Finally, there might be handling issues that appear by having D range tires.

The reality is that you bought a 3/4 or 1 ton pickup.  A part of owning a 3/4 or 1 ton pickup is putting up with a rock hard ride with E range tires.

Thanks for some of the replies. I could understand it on a truck since they could load up tons of dirt/rocks/bricks/metal/etc. in the bed. The Suburban uses the same chassis so I was figuring they just made an across the board requirement for the chassis without regards to the body that was put on it.

My 03' Mustang GT requires Z rated tires (for speed)... in case you were wondering so does the Lamborghini Aventador... I think the question is genuine.. I happen to put Z rated tires on my stang, but when I look at the actually requirements rated by the tire industry I don't need to.

I do understand there are differences within the Z rating.. but my car is not going in excess of 149mph and the general Z tire can go beyond the 149mph mark.. etc

My last set of D's were rated for the same weight as the E's that came off of it. If that's the case, I wouldn't worry about it.

YMMV

ETA: Honestly, the old D rated tires felt better towing than the current E's.