Hoss,
I found with my truck that it wasn't the knob, just the little insert that shows the shift pattern, as Thomas says. I could re-align it by twisting it back into place, but it would turn again.
I finally solved this problem by prying it out and putting just a SMALL AMOUNT of RTV silicone glue at one spot on the edge. I haven't had to remove it since gluing it down, but I'm pretty sure that I will be able to pry it out even with the glue in that one spot. I suspect that you could probably pry the knob away from the insert and push some glue into the gap with a toothpick without even removing the insert.
Mine hasn't turned since.
Loren
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2000 Quad Cab, 4x4, short bed, six-speed: U. S. Gear exhaust brake, MileMarker Select Drive viscous coupling, Velvet-Ride spring shackles, Painless Wiring circuit box, Optima yellow-top batteries, Super-Glide 5th wheel hitch, Reese folding ball gooseneck hitch, Drawtite front hitch receiver, Tekonsha Sentinel brake controller, Westin nerf bars, Clarion sound system ***
1995 Standard Cab, 4x4, long bed, automatic: Firestone Ride-Rite air springs, torque converter lock-up switch, 4" cat-back exhaust, Reese 20K 5th wheel hitch, Reese gooseneck hitch, Hayes Micro Control trailer brake controller, 100 gallon bed-mounted fuel tank with 12 volt electric pump & fill nozzle, Swiss Cap fiberglass cap, ICI chrome nerf bars ***
Towing: 1995 Avion fifth-wheel (33. 5', 14,000 lbs) (belongs to a friend), 1996 Holiday Rambler (32', approximately 6000 lbs), 16' flatbed, 34' gooseneck race car trailer.