I'm an artist and I do need to take photos of my paintings with heavy texture. I bought a 50mm f1.8 with my new Nikon D800 and I find some distortion when the painting is big 36"x 60". I used to take them with my canon T2i with a 50mm F1.8 and I didn't have as much distortion. I know the Nikon is a full frame so should I try a different lens to not have as much distortion?
There's a somewhat similar question here, although it was more geared towards which camera was better I still think there's some value in looking into it for other misc info.
As for distortion, pretty much every lens has distortion, the difference is that it's sometimes a bit more obvious in certain lens due to its design. Fortunately something like this is easily fixed. Correct it in software. There's no shortage of software that helps you do this, Lightroom, Gimp, Photoshop, etc.... In Lightroom there are presets for most common lens so it makes it a snap to correct.
You could also use a different lens, probably check out some reviews online to find out if the lens has lot of distortion.
You have a D800 which has enormous resolution, so I think the way I would tackle this is to pull back a little and crop the result rather than filling the full frame. Your 50 f/1.8 was designed for a full frame camera (if I recall correctly) which means that on the T2i, you were hitting the lens center sweet spot. That's basically what I'm suggesting with the D800.
Bear in mind too, with the crop, you're still likely to end up with images that are significantly larger than your old T2i. That's been my experience since switching from a Pentax K-5 to the D800, I've done what I've thought were aggressive crops and the results were still larger.
Your other option is lens correction. You don't mention your conversion software, but Lightroom and Adobe Camera Raw (from Photoshop) offer this, as do others. Despite being a fellow D800 owner, I have not used the Nikon software so I can't comment on that. Having said that, software correction is never going to be as good as having it right optically.
I also have to make mention of a shooting technique to reduce the distortion to whatever the optical design presents by ensuring you have a PERFECTLY level camera in a perfect parallel plane with the artwork.
An extreme example is tilting your camera upwards and shooting a skyscraper. It makes those buildings look very tilted.