You could put a single speed kit on it and remove the front brake, you could put some rigid forks on it, S&M do some. It will still be an mtb though
And why bother, the gears will help you pedal with less pain and the suspension will absorb a lot of the jolts that hurt your hip
If you want to try a cruiser, I'm sure one of the troops would lend you one for one of the rides out