I had similar problem last winter. When I swtched to 4H the light continued to flash, when I swithed back to 2H the 4x light stayed on. I finally stopped the truck and put it in neutral, switched to 4H then went into reverse. I heard a dull thump and put it into drive and moved forward, the light stayed on so I stopped and switched back to 2H and moved forward. The light went out and I have not had any problems since. I can only assume something got hung up or jammed.