First Place to start is the air system and valves. The air is feed into the pressure regulator at right rear top of transmission, there are two range cyliners on the rear section.
The lower one on left is the splitter, the one on the top right behind the regulator is the range cylinder. The valve on the left side near the top/ middle is the slave valve.
Sounds like the air system is dry, remove the air fed line from frame to pressure regulator at rear of transmission, get a bottle of air tool oil and squirt a good amount down the
air fed line once it stops flowing air. Reconnect, build air and operate the low range and splitter back and forth several times. If it was dry it was also/probably shifting slow.
With the air tool oil it should speed up the shifts. Normally this symptom happens when the slave valve is dry. Moisture in those air lines is not good, water plays hell with
that air system. Has the air dryer been serviced recently?