I have a MF54A backhoe attachment, its not mounted on anything yet but I have looked at it and think the boom has to be removed from the base to get the swing motor out.
It has a tab that keys it to the base and then after you have the boom off the base it looks like you remove all the hyd hoses off the top of the swing motor and then either when you pick the swing motor up a little it comes free of the boom or it has a big pin from the bottom that secures it to the boom.
I have a book that came with it, I will look at it when I get home from work and see if I can tell anything from that.
I just took the hoe up to my property 30 miles from here the other day, will be back up there in 4 days or so.
Im thinking that when I mount up the hoe that I may just remove the swing motor and mount 2 hydr actuators instead for the swing, to avoid the situation you are in. My boom is not attached to the base right now.