Steering gear box???

Sounds to me like something is binding, most likely steering column u-joints or slip joint.

 

Sounds like the joint within the column needs to be replaced and it going to fail, don’t use the truck until it is fix, you will get into a serious accident and kill someone or a few people.

 

You’re for sure in the ballpark.

I put a new box on my W9 about 6 years ago, about $700….

 

Had to replace mine on kw t660, the same price, i just use same oil as engine.

 

The occasional clunk you hear. Does it happen with the steering stiffness. IF the clunking doesn't happen at the same time as the stiffness. I suggest the clunking noise may be the front axle leaf springs moving against each other. Not a big deal. Unless they are cracked. Or if you see rust trails around the U-bolts, they may be loose.

IF the stiffness was not present before you changed filter and fluid.
I would flush the system and remove the filter housing to clean, replace the hoses and filter(again). Call a freightliner dealer for correct filter and fluid.

Good luck

 

Found this:
Heavy-Duty Trucks Maintenance Manual (freightliner.com)

46–03
TRW Manual Steering Gear Lubricant Changing, Model 503
NOTE: In climates with extreme temperature changes, lubricant should be changed every six months.

1.
While the steering gear lubricant is still warm, remove the steering gear drain plug and drain the old lubricant. Lubricant will drain faster if the fill plug is also removed. See Fig. 4 . After all lubricant has drained, install and tighten the drain plug 14 lbf·ft (19 N·m).
WARNING
Use only an approved lubricant. Failure to fill the steering gear with approved lubricant could adversely affect steering ease under certain conditions and could hamper vehicle control. This could lead to an accident resulting in injury or property damage.
2.
Fill the steering gear with clean lubricant to within 1/2 inch (13 mm) of the fill hole. Use hypoid gear oil API service classification GL–5 (with no zinc additive). At operating temperatures continuously above 30°F (–1°C) use viscosity grade SAE 90. At operating temperatures continuously below 30°F (–1°C) use viscosity grade SAE 75.
3.
Install and tighten the fill plug 14 lbf·ft (19 N·m).

 

This.....

 

Yes, they are called "splines".....