Regarding your question on tools to remove the track, this article (Natural Handyman - How To Repair and Adjust Pocket Doors) says:
Many pocket doors can be adjusted with
special tools from around the top of
the pocket frame with the door in a
partially closed position. These tools
are often specially angled wrenches
designed to fit into that little gap.
These tools are part of the assembly
kit supplied with the pocket door, but
are rarely given to homeowners by the
builders. Without these proprietary
tools, you have to improvise with or
modify standard tools.
If you can't get to the warped part of the track with any tool that you have (or can find), then you will need to cut a hole in the wall for access.
This article (Ask the Builder - Fixing a Pocket Door) has a great description of how to get access to the track. The article talks about replacing the track but it also gives a warning that if the new track is not the exact same dimensions then the little project will become much bigger. So you will have to make a decision to cut the wall to get access to the screws to replace the track or just to get to the warped section of the track.
A quote from the article:
Remove the door from the pocket and
take it off the track. Use a
flashlight and peer up into the pocket
cavity. There is a good chance you
will see the screws that fasten the
track to the frame. It is now time to
cut an access hole on one side of the
pocket wall. This hole will allow you
to get your hand or arm into the
pocket so that you can loosen and
remove the screws and subsequently the
track.
I had basically the same situation in my 70's-built house, the closet doors went all the way to the roof, and were horrible looking bi-folds.
Basically what I did is framed in a header to bring the opening to 81" (remember to account for 1/2" drywall while framing):
Then some drywall and paint (note I didn't yet repaint the ceiling in this picture):
I had 4 closets like this. One by the front door got sliding mirror doors, the rest got frosted-glass sliding doors. The right picture below is actually the "after" shot of the first one in this answer.
The width is going to be challenging for stock stuff. If it was a bit wider, you could probably split the door into two by putting a small column in the middle, but that would leave you with a couple 3.5' doors, which are pretty small. If you went to bi-folds, you might be able to do a 4' and a 2' door, though that might look a bit strange.
A quick search turns up some custom closet door manufacturers, which might be an option to get multi-panel sliding doors, like this:
If you do this you can probably also get them 8' high, but honestly, even without the doors on, just making the opening 81" really updated the look of the house.
Yet another option is that you could shrink the width down to 72" (basically just extend the wall/framing on one side by 9"), and along with making the height 81", you're into a standard size where you can get both sliding or bi-fold doors off the shelf from any box store.
Good luck, and let us know how it turns out
Best Answer
I may have led astray with my photo as the upper track was unrelated to the issue.
It turns out that the peg+plastic sheathing was very tight when at one side of the lower track. Why? The only possibility that I can make sense of is that the door/track itself has contracted on that side. Putting a touch of lubricant just here helped an enormous amount.
Thanks all.