Since you are using an MTB you will be more upright compared to a road bike. I found out that my back pain is not as bad when I'm less upright on the bike. By being more perpendicular with the road the pressure on your lower back gets evenly distributed to your entire torso.

+1 on getting bar ends for your handlebars. I used to experience hand pain and sometimes numbness when just holding onto the standard straight bars. This is why road bike handlebars are designed the way they are. They fit more naturally on peoples hands especially the hoods.