2001 ford explorer.....In park it won't sit still on a hill. could this be linkage and how much to fix?

If it starts in Park it's not linkage because it is going in to Park. More like the dowel that freezes the transmission in Park is broken. How much to fix? Hard to say, they might hit you up for a $1500 transmission

Eric the car guy takes an automatic transmission apart and shows the paw that locks the car in park just after the 15:00 mark. Your transmission is different but the mechanism should be similar. https://www.youtube.com/watch?v=JGbsgpp2YJQ

i would not take a transmission apart if you dont know what you're doing take it to a professional, if you try yourself it could just get worse. it would be cheaper in the long run....i agree with david tho

use the parking brake and save some $$$. Buts its most likely the dowel pin that broke.

That is an option Andrew, especially if she(?) (rox?) is on a really tight budget. Parking Brake... and turn wheels to curb... and even carry a chock, if otherwise it runs and shifts properly.

The parking pawl inside the tranny is either damaged or not engaging. While not familiar with your particular tranny, try this. With someone inside the vehicle shifting the gears through the entire rance, locate the lever on the tranny which shifts the gears. It will be moving. When the lever is placed in the park position, note where the linkage moves, there may be separate lever on the tranny where the linkage engages the parking pawl. If it is lever problem, it can be fixed from outside. If the pawl itself broke, which is only a single toothed gear, the tranny will have to come out of the vehicle. I've seen individuals damage the pawl by parking on inclines, then when returning to vehicles, using both hands on the shift lever to force disengagement. When parking on an incline, if possible, turn the wheel to the curb and bring the vehicle to a stop against the curb before engaging the parking pawl. This will take the weight of the vehicle off the gear. You can also position the rear tire against the curb if parking uphill.

In my opinion that is an excellent answer OJ. Well done, good advice. But as I said above, it would not hurt to carry a wheel chock too.

What is a wheel choke?

wheel chocks are wedges put under the wheels to hold vehicle in place so it won't roll away.