Truck Repair: Transmission, valve body, 4wd

Question
I have a 1995 Chevy 2500 PU (350 eng)(4WD).  All of a sudden the 5SP trans has lost 4th gear and there is a wining noise.  What happened and how can I fix it?

Answer
It is most likely a valve body issue --- you can drop the pan and change the trans filter and fluid --- but when a transmission is worn to the point of losing gears -- the overall condition is probably poor. A rebuild is right around the corner.