Suggest posting a screenshot that shows the banding and provide camera and file type (raw, JPEG) you are using.

I've been using the profile 'Camera Neutral' for years as I really like the look

I remember early v1 profiles for some Nikon to have some problems in

the shadows. The are v4 profiles for the D300 in Lightroom 4 — try

those.

There is an issue with the 'local tools' in that they are applied before the global tools, which can reduce dynamic range in the highlight and shadow areas. This may also be the case for the Post-Crop Vignetting tool, but I haven't tested it.

http://forums.adobe.com/message/4612100#4612100

Why it only shows up with certain cameras is probably due to the available dynamic range in the individual raw image files.

Another possible issue that could be contributing to the "banding" artifacts is your Dell 2408WFP wide gamut monitor. Using a wide gamut monitor with 24 bit display data stretches the available gradations (16,777,216 colors) over a wider range, and provides less smoothness (i.e. banding). Many wide gamut monitor users will argue that this is NOT the case for them and that may be true. But anytime you use "extreme settings" in LR (100% settings, two graduated filters/spot tools) you're setting up a situation where banding may become visible. Try exporting an image with banding using TIFF, ProPhoto RGB profile, and 16 bit data, and then view it on another system equipped with a standard gamut display. You can also download this Ramp test file from AMD's site, import it into LR, and see if you have visible banding:

http://www.amd.com/us/Documents/ramp.psd

Photoshop supports a full 30 bit display data path when using a 30 bit graphics card, but Lightroom only supports 24 bit display data.

In short you have multiple things in LR that may be causing the artifacts you are seeing. My best advice is to use the global controls more heavily (Highlights, Shadows)  and lighten up on the locals, since they can reduce the dynamic range in your image.

Have you setup the monitor using a hardware calibrator, and if so what calibrator and settings for Gamma and Luminance?

Then what you are seeing is "stair-stepping" or "posterization" due to LR's 24 bit display output to your wide gamut 30 bit display. It's one of the shortcomings of using a 30 bit display with applications that only support a 24 bit display data path.

http://www.cambridgeincolour.com/tutorials/posterization.htm

The same thing can occur with standard gamut sRGB images as explained at the above link.

The banding most likely will not appear in exported sRGB profile output files or in actual prints. If it does you can minimize the banding by adding noise using the LR Grain tool, or in PS using 'Add Noise.' Sometimes lowering saturation and/or contrast in the affected areas with the Graduated filter and/or Spot tools will reduce banding as well.