Hair's dealt with dogs with joint issues. Not sure about a slipped disc.
Sorry - I've never dealt with exactly that. I've certainly seen the dogs in the little carts on TV. I'd do some very careful thinking and talking with your vet.
When my old labrador Logan was 16, his back gave out and paralyzed him... I thought it was profound how he was obviously in no pain, but he was immobile. I put him to sleep then, but he was paralyzed both front and back and there was little else that could be done.