The column shift mechanism is probably getting worn out, pretty common issue on column shifters.
Disconnect it at the transmission and shift the transmission levers underneath the truck, and see if you still have the same issue.
Have somebody else shift the linkage in the cab, while you watch the linkage in front of the firewall, see if you can tell what its binding on, its likely wobbly worn linkage holes.
If its the transmission levers hanging up then its worse, its the syncros in the transmission.
