I seemed to have solved the problem. My mistake was allowing color management in the Print module of LR4 to be "Managed by printer." I should have selected the printer profile directly from Lightroom. Thanks to Phil for pointing that out.
Another interesting finding was that this was the only parameter that influenced the look of my images. As long as I allowed LR4 to select the printer profile, anything I printed looked good. This includes images files without an assigned color profile, or sRGB, Adobe RGB (1998), or ProPhoto RGB. (Incidentally, before this exercise, I didn't realize that ALL of my Nikon NEFs have the Adobe RGB (1998) color profile. This included the D100, D300, D90, D7000, D700 and D3s.)
For anyone who wants to know, I printed four images in a test mode, and here are the inputs and the results:
Image #1: Lupine meadow
bit depth: 8-bit
file format: PSD (from scan of slide)
color profile: none
Printer driver controlling color via custom printer profile: dark, slightly more magenta result
LR4 controlling color via the same custom printer profile: looks good, matches the calibrated screen
*I converted this image to a 16-bit TIFF with a ProPhoto RGB color profile, and the results were the same
Image #2: Bristlecone pines
bit depth: 8-bit
file format: JPEG (from Nikon D3s shooting in JPEG mode)
color profile: sRGB IEC61966-2.1
Printer driver controlling color via custom printer profile: dark, slightly more magenta result
LR4 controlling color via the same custom printer profile: looks good, matches the calibrated screen
*I converted this image to a 16-bit TIFF with a ProPhoto RGB color profile, and the results were the same
Image #3: Mount Whitney
bit depth: 12-bit
file format: NEF (from Nikon D100)
color profile: Adobe RGB (1998)
Printer driver controlling color via custom printer profile: dark, slightly more magenta result
LR4 controlling color via the same custom printer profile: looks good, matches the calibrated screen
*I converted this image to a 16-bit TIFF with a ProPhoto RGB color profile, and the results were the same
Image #4: Nevada Fall
bit depth: 14-bit
file format: NEF (from Nikon D7000)
color profile: Adobe RGB (1998)
Printer driver controlling color via custom printer profile: dark, slightly more magenta result
LR4 controlling color via the same custom printer profile: looks good, matches the calibrated screen
*I converted this image to a 16-bit TIFF with a ProPhoto RGB color profile, and the results were the same