Sculpting the model added geometry. Decimate took some away. Even if the weights attached to the armature were correct to start with it's very unlikely that they would still be. In order to make the armature useful again the weights need to be repainted.
Switch the BearSkin to Weight Paint mode (Ctrl + Tab).
In the Outliner panel unhide the Armature.
Select the Armature in the Outliner panel and insure that it is in Pose Mode.
Insure that the Pose Position button is selected in the armature object data tab.
With the BearSkin in Weight Paint mode and the Armature in Pose mode/Pose Position, it is time to fix the weights.
Similar to sculpt mode, the weight paint mode has a set of brush tools, but really there is only one brush necessary. The draw brush has a blend option that duplicates all the other brushes and is considerably easier to find the tool desired.
Select (right-click) the BearSkin.
Verify the Weight and Strength are
The procedure is as follows:
Select a bone...