Transmission Question

Have an MDX and when driving between 30-45mph, RPMs sit between 1-2. Sometimes
when I give it a little more gas to get up a hill, it will stay in the same gear, under 2,000
RPMs, but go back and forth between 1,500 rpms to 2,000 rpms. The engine kind of
shudders. This only happens on occasion, and if you give it more gas (not floor it, but a
little more) it will switch into the next gear, and go up to 2,500 rpms and have no problem.
Is this something I should worry about, or is this just how the tranny ratios are set up? I
have also noticed that when you move the gearshift from Park to Drive it takes about 2
seconds for it to engage into park. If you let off the brake before that, and hit the
accelerator, it will shudder a little and then go on its way. I am used to driving a
Highlander, so I know the drive will be a little different. Just didn't know if this was worth
worrying about, or if this is typical for this MDX. It is a 2008 with 94,xxx miles. I do put
premium gas in the vehicle.