Can you give more detailed description?
One problem I had with my 6 speed was that the joint on the shifter got loosen bolts and when 1 bolt come out, the shifter would not shift the gears. Pull the floor boot and check if the bolts are holding?
As far as I can tell, both reverse and low have some level of synchronizers. They don't seem to work as well as the synchro's on the other gears, however. It's a lot more difficult getting the tranny in reverse and low than it is other gears. Especially when it's cold.
That's just my experience.
Edit: Yes, according to the parts explosion diagram, Reverse-Low is synchronized.
It is not a clutch issue.
I would suspect an issue with the linkage or transmission. Im not sure but i believe the shifter enters the transmissin from the top so it is either the stick is binding internally or an issue with the shift collar. You may need to have the tranny removed.
Not sure on the older trucks but i know the newer trucks Low gear is non syncronized.