They could easily have done a traveling matte to replace wheels with a clean background (the camera doesn't move for the shot where Luke's looking around the corner, the only time we see it floating completely in frame).
Most likely a blank plate with the wheeled rig just masked out. Nice and smooth and easy even at the time.
Well, it did not work only three years earlier... Remember the Landspeeder?
As said above, locked off shot means you can shoot a clean slate to mask over. As I said, easy at the time. Personally I don't think wires would have been as effective and would have wobbled. Good size soft wheels would have done the job nicely. However only the guys who were there know! Either way would potentially work.
While shooting a clean plate would have been easy enough the compositing over would have been more difficult, remember, they didn't have motion tracking technology back then and even though the shot was locked they still would have to have tracked the motion of the actors and the rig to the background. Remember, they were still dealing with optical composites back then and even something as simple as covering up wheels would have been tricky to do back then and I think that it would have looked comped like a lot of the comps did back then.
That's called rotoscoping and ILM was doing that all the time. A hand-drawn matte, frame by frame. Like here, just a year later:While shooting a clean plate would have been easy enough the compositing over would have been more difficult, remember, they didn't have motion tracking technology back then and even though the shot was locked they still would have to have tracked the motion of the actors and the rig to the background. Remember, they were still dealing with optical composites back then and even something as simple as covering up wheels would have been tricky t
Something else just came to me that makes me think that it was wires or something simple like that and that's the old ILM book that came out decades ago, the big hardcover book that talks about them from their inception up to Star Trek III. In it they show lots of pics about how they did various effects shots for everything they had worked on at the time and there's no mention of the Bespin scene, if they did something fancy in post I would think that they would have mentioned in the book and shown how it was done. Then again, it was not like the book had a break down of every effects shot they did in every movie they worked on either.