Thanks for your reply.

To clarify:

P - Works
R - Works
N - Works
D - Works in starting gear (2nd?) - but won't upshift to either 3rd or 4th.

Upon reaching somewhere around 4K rpm's, the transmission disengages until it is switched into neutral and back into drive, at which point it resumes its "stuck in 2nd" mode.

This is what I have read on various forums and posts is known as limp mode. I believe that there are two limp modes, one for transmission and one for engine. It sounds like you were referring to the Engine limp mode, which is not what is taking place.

Thanks again.