Help with transmission....Please

I have a 2001 Focus sedan /w 100K on the miles. I've had a little
trouble with this and I cannot quite figure it out. I think it may be
a transmission range sensor but I'm not sure.

The car seems to shift fine while driving but I can shift to R and all
seems fine. But if I go from D to R the car seems to be in N. N will
be drive. I have to go back to park for all to be normal again.
Then once again in D and all goes haywire.

Any suggestions would be appreciated.

Thanks,