General Question 2002 Ford Explorer

I am about to hit 139,000 miles on my explorer, and so far no major problems (knock on wood, all I have had to do in the past 50K miles is new tires, brake pads and an oil change every 3 months) I have, however, noticed that, on a cold morning, it sometimes stalls out (but always cranks right back up) and when I am parked on a hill (facing downhill) it doesn't like to shift into "D" (it requires me throwing all of my weight into shifting it into gear!)  

Is this a preface to future transmission problems? I really love my car, and aside from some cosmetic defects (cracked rear panel, which seems to be a reoccuring problem) she's in great shape! I am just a little fearful she's going to klunk out in the middle of Atlanta traffic, or on the side of a busy interstate!!