Consult your Doctor before you try anything.
I assume since you mentioned knees, back and neck, we're not talking any actual paralysis so much as vertebral/disc problems and such.
Depending on the severity and location of your back problems, you can probably do upper body exercises.
Some ideas you can talk to him about would be upper body weights or arm peddle machines (again, no clue how/where your back is hurt so you gotta run it by the doc).
If weight loss is your goal, you might want to start with diet modification vs adding actual exercise (again, talk to the doctor who knows your precise situation),
Also, are you in physical therapy? Those guys can usually recommend some really good things to do ^_^
| current weight: 150.0