Another test you can do is drain the trans oil and if it still goes forward, trans must come out.
5FGCU25 is mechanical linkage shifting with a plunger switch in the control valve for neutral start inhibit function.
No solenoids or directional switch.
get a pressure guage and check pressures at the control valve
also check your circuits at the solenoids to make sure both of them are functioning and check the signal from the directional switch too to rule out that part.
yes it very well could be a mechanical issue inside the clutch packs but rule out the obvious first before tearing the tranny apart.
Yes, forward clutch pack has likely failed in that the large retainer ring that holds all the discs and plates in the drum has let go on one side.
You can check that by removing the trans control valve assembly (aka "valve body".
The truck does not have reverse mode because when in reverse, the forward clutch is still applied, and so the reverse action is cancelled out completely.
Sounds like the forward clutch pack has an issue.