I put a new cylinder and seals in my D6c recently. Wasn't too bad.   Not sure about the D6D but before you break the track you need to compress the track spring a little and put some spacer blocks in between the stops and the spring.  (just pump grease into the adjuster until it compresses the spring)  Otherwise there wasn't enough room to remove the track adjuster cylinder.  Only took about 1/2" on mine to give enough room.  You might want to call a shop and ask if it's necessary on your machine.

Other than that it's not too bad....break track, unbolt push rod on idle yolk,  roll idler forward, and pull out cylinder.

I'll dig out my manual and see if I can scan a few pages and post them.