The quickest fix I can think of, but would only do as a last resort, is to drill a hole in the bottom of the bracket, into the mirror housing, and put a screw in there. Make sure you don't hit any needed parts inside.

Chris

 

My '97 1/2 ton had the same problem. I took the mirror off the truck and found that the mirror shaft has a flat spot on one side (think of a piece of 3/4 round moulding). It goes through a hole inside the bracket that has a metal plate on one side. In normal mirror position, the flat on the shaft is in contact with the metal plate (the metal plate screws into the plastic bracket). When you pull the mirror in, you rotate the flat off the plate (the plate flexes a little). This is all easier to see once you get it removed.
I fixed mine from the inside. Made sure that you could not rotate the shaft anymore. I JB welded everything inside so it wouldn't move. Was cheap. The only drawback was that you couldn't pull the mirror in for drive-thrus etc.

Hope this helps,
Rob

 

Have you checked with your local truckwreckers and salvage yards? Maybe you can find some of the plastic 98 + mirrors. The look better in my opinion and dont seem to jump around as much.

Up here some of the local papers have trucks that people are parting out. Sometimes you can get the parts for reasonable. May be worth a try. Good luck.