Hey Dan,

The ring finger is definitely difficult to separate from your pinky.

Two things you can try:

1 - You can just practice controlling your fingers separately without your guitar. Just hold your hand up like you're waving hello, and try to move each finger individually. One at a time, folding it down toward your palm.

2 - When doing the D5 to D6 thing, try it way higher on the neck first. For instance, at the 9th fret (it will no longer be D5 and D6, it will be F#5 and F#6.) But there's less of an intense stretch. Once you get more comfortable with the shape / movements, bring it back down the neck to D. If you're on an electric you can even do the same shape on the B and high E string, even higher on the neck. Then work it back down.

-Carl.