Have you ever done it? need rollers and the track fixed. the rollers don't worry me, but how do I fix the track?

you don't, the track is part of the whole frame and in all the yrs I have installed them/fixed them track has never been available.
best bet is put in a new door once the track goes.
there is another tff'r in arlington I think that needs one taken out and french doors installed. maybe pm him about his old one for cheap if its the same size as yours. his is 5'x6'8" normal size door.

is it hard to replace? I'm pretty handy. if the frame is the same size, put it in and shim it till it's square?

basically that is it, you have to have the door closed and locked, make real sure of the reveal on the operator side (it must be the same on the strike side all the way down).
if it was me though I would just go buy new if ya can, much easier to deal with.
lowes has some french doors that are not all that much either and much easier to install.

http://www.lowes.com/Windows-Doors/Doors/Patio-Doors-Accessories/_/N-1z11ppg/pl

the relia built at the bottom of the page or one of their french

I just replaced the sliding door in my rent house with a french door from Home Depot. With hardware a metal door was under $400.
I had to cut the drywall out of the opening as the slider just came partly into the opening. I had to install casing inside and brick mould outside to trim it out. All in with trim was about $450.
It took me and a helper about 2 1/2 hrs. Still need to paint it.

Now I don't want to mess anyone up with New Details, but there is a way to fix that track also. Yea, get your rollers at Home Depot but you can order a "Patio Door Sill Cover"...I like to call them "Track Caps" from your local glass company who can get them at CRL.

Stainless steel 8 footers cost about 8 bucks and goes on top of your bad track like an upside down horseshoe. You cut the length, put on track and crimp the cap...and go fishin'.

Most glass companies won't sell you a six dollar item when they can install a completely new door for $1,600.00. But the track cap works and if your door has been with bad wheels and track for a long time be very carefull. You would be surprised how many I have fixed, and the customer wasn't ready and opened the door with the usual jerk, and slammed the door open (or closed) so fast, it broke the glass.

Track Cap

Home depot has the track cap as well.