All right, here's all the differences between the various 5.1 versions:
All of them are primarily derived from the 1993 PCM tracks, but with certain edits and changes according to whatever I was trying to accomplish with each one. These edited stereo tracks were upmixed to five channels and combined with LFE bass from the special editions. As such they have a very authentic feel, intended to be reminiscent of the 70mm theatrical releases, though some of the specifics are not the same.
SW's 5.1 is the closest, because there are in-theatre recordings of the actual mix available, and the 1993 version was sourced directly from the 70mm printmaster, albeit with additional sound effects layed on top of it. I removed these extra sounds using the 35mm stereo soundtrack, while most of the LFE came from the 1997 5.1 mix, with some 2004 and 1993 bass in places where those seemed to work better. Each bass effect was individually level-adjusted, and sometimes also in synch and duration, to provide the optimum blend with the main channels. It's not perfect, because the 35mm replacements are less dynamic and have narrower imaging and lower fidelity than the '93 mix, but I did my best to make it all sound like one consistent track without any jarring leaps between sources. How well I succeeded depends how good your ears and audio equipment are, and how picky you are about such things, but overall it sounds rather good, if I may say so myself. ;)
The other two movies don't have 70mm recordings available, and their '93 mixes were made somewhat differently than Star Wars, so direct reproductions were out of the question. Because of that, I wanted to make 5.1 versions that sounded best to me. For ESB, this meant inserting the snowspeeder crash sound from the 1980 stereo (missing in the '93), and replacing Boba Fett's departure from Bespin with the 1997 mix, since the music cue in the original was hacked up in order to end when Chewie begins firing at the ship, while the SE used the cue as composed until the scene ends. I also repaired two distracting glitches in the PCM. For RotJ, I inserted the deleted music from Obi-wan's appearance on Dagobah, since to me the scene is greatly improved by its presence, and used the 35mm stereo for Vader's defeat since the music is heard more prominently this way (which also fixes a glitch heard in this part of the '93 mix). The LFE channels came from the 2004 mixes and were again level- and synch-adjusted for the best sound. These edits comprise the 'version 2' 5.1 tracks listed above.
Not everyone wants to hear the movies with all of these changes, since some of them are in fan-edit territory, so by popular request the 'version 3' mixes were made, and will be used by d_j along with the SW 70mm on the upcoming dvd's. For the most part they are exactly the same, except the music edits for Fett and Obi-wan were removed and reverted to their 1993 forms. The 35mm inserts for the snowspeeder and for Vader's defeat are still present, as are the glitch repairs.
The 'version 1' were made a while back and were rougher attempts at what I've since perfected in the later mixes. They used the 1993 tracks without any changes, along with the 2004 LFE; but the bass editing was not as good as what I managed to do later. Mostly they still sound similar, but the 2nd and 3rd versions give much more pleasing results.
The actual upmixing and encoding to ac3 was done by Satanika, since I could not get any upmixers to work on my computer, and I'm grateful to him for helping me out with that. :)