can you spin the clutchbell on the truck? gear mesh isnt too tight? my MGTs have a OWB in the trans that if pushed forward the spur wont engage, but pushed backwards it spins the spur gear so if the clutchbell is locked up, the spur wont spin not allowing getting pushed in reverse
if your gears spin as they should. it may be taking apart the trans and making sure everything is as it should be.