You can replace the rear shock if you like, but if the your shock is not bouncing (blown or leaked out), then your probably still in for a rough ride. They only have around 5" of travel (4.9" exactly if I remember correctly). There is only so much comfort you can get with so little travel.
The front suspension uses struts on the Timberwolf. You cannot replace just the shock portion. The entire assembly must be replaced. Unfortunately there is no aftermarket replacement for this. You can only replace it with the same OEM part.
The Timberwolf is not a good riding ATV. None of the Yamaha utility ATV's from that era are or were. Actually, about the only soft riding utility ATV's from that era were Polaris, and they had notoriously poor reliability in those times. Worse than today.
You can install a different, suitable rear shock if you like. Get one that measures close to your factory one. You could also install a slightly longer shock to possibly get a little more travel. But be weary of doing this. This puts the rear U-joint in the swingarm in a sharper angle, increasing wear and possible breaking under heavy load. And those yokes are stupid expensive.