I'm updating and moving this over from the mothership. When I first got my 89 Wagoneer the rear door arm rests were shot. This made closing the rear doors from inside a real chore since the later wags, I beleive 89 and up dont have the door pulls anymore. Like most of us on here I searched the parts section and bought a few arm rests from fellow members. Some were good, some were just as bad as mine and some were worse. After I bought one from Florida that looked and smelled like it came out of a swamp, I had had enough and decided I was going to come up with a solution to fix the ones I had.
Root Cause
Here's what broken armrests look like when removed, the holes in back are broken up and the screws go most of the way, if not all the way through.
The above holes have verry little to do with holding you armrests in place, the real problem lies underneath. We need to take apart the arm rest to see the true problem. Remove the 4 screws holding the back piece on and gently pry the back off the armrest. You should end up with something like this
Here is the true problem, you can see in the above pic the plastic cup which holds the head of the fasteners are no longer intact. This is caused by over tourqing the fasteners(I'm guilty of this myself). The right side one is completly gone and the left is not far behind, there is still one post on the left side(see below). These posts contact the back plate and give support to the cup and help distribute the tourqe.