'93 gmc Sonoma stuck in reverse

Driving home this afternoon in our 93 GMC Sonoma, which
probably has 400k km on it, and going up the final hill to our
destination on the highway, going maybe 80km/hr, the truck
felt like it was needing to shift. I went to shift from. 5th to
4th, and couldn't. Nor could I go into any other gear. I
immediately pulled over, tried all gears again with no luck.
The truck was still running absolutely fine, and then when I
tried again, it went into reverse and that was that. Stuck in
reverse.
Clutch is not limp at all. Truck runs fine. Simply cannot
change gears and is stuck in reverse. Is this a cracked
driveshaft? Nothing is hanging down under the truck
abnormally. Truck leaks oil as it is, but other than the
normal droplets there was no liquids exuding from the
vehicle. I shut it off and called my father in law, but I'm just
curious on some opinions before I send it to the wreckers.
Driveshaft?
Possible bent or broken pins?
Transmission?

Any help is greatly appreciated. Thank you!