There are several reason why this could be happening. There could be a linkage broke, a bad pump or just a bad transmission in general. Most likely the pump is bad or the transmission is bad.
You are probably low on transmission fluids
truck will only go forward not in reverse does this mean the transmission is gone
Your transmission needs to be rebuilt ... although - once - I "fixed" that problem by draining & refilling my trany.
In the Model T transmission, each of the gears does the same work. The clutch is only used during a direct drive. The Triple gears are used to drive in low gear and reverse.
you have a problem in your torque converter or transmission pump
Time for a rebuild.
For the Reverse problem, you could have low transmission fluid, or a bad transmission. For starting in drive, just about every modern car will not start when the transmission is in gear. They will only start in Park or Neutral. This is a safety feature to prevent the car from taking off into someone or something when started.
transmission leak in reverse only
That you need a transmission shop. It is possible but unlikely that your linkage needs adjustment; more likely is that you need a new transmission.
Brakes locked?
linkage broken, bad pump, bad transmission. best guess is bad pump or tranny. hope it helps. duboff
If it is computer controlled it may be in limp mode. That happens when the computer detects a malfunction and sets a code.