lol i understand where you're coming from - i'm only 24 and an absolute physical wreck.

for now. i've been lax on my workout regimen. and that's my advice for you. built up enough muscular strength to shore up the failed(or failing) joints.

for the feet i recommend calf raises. stand on a stair, on your tip toes, and raise your body weight all the way up and down. do it till you can't, then wait a few minutes, and do it again. and so on as your tolerance builds. remember really important you have to stretch the hams a couple times a day or face excruciating cramped calves the next day. you still might but it will eventually increase the range of motion and strength down there.

for knees most non-impact exercises work great. biking, squats, and lunges are all really good for the knees.

for the shoulders - and i've got the dislocation too ^.^ - i like pull downs on a pulley machine, french dips, push ups, and finally pull ups and dips. in that order over time. start with one, and when that's no longer challenging add the next one, and so on. over time this will build your body's stress tolerance back to levels where you can handle more physicality.

also core exercises or you'll lose all the work on the limbs =>

it's a long process. feel free to pm me for more info if i need to clarify anything
-matin