A "broken axle shaft" will not result in one wheel turning.
A broken axle shaft will result in NO WHEELS turning because a conventional differential requires that there be traction on both axle shaft ends to operate and provide movement.
A broken axle shaft will cause all motion to be concentrated to the output side of the differential where the broken (or missing) axle shaft is located. Or, a similar scenario can be created simply by removing either axle shaft from the truck. When you do that, the truck will not move under its own power because all the differential parts are free to turn uselessly inside of themselves.

The more important aspects of trying to determine why this truck only wants to spin one wheel can be tested in this manner.


And beware that you are doing tests where the vehicle might suddenly attain traction and thus move or jump off of a jack or a wooden block, so keep everyone clear from standing in front of or behind the truck.

With wheels off the ground, can you easily rotate both wheels by hand? If so, when you rotate one wheel clockwise, does the opposite wheel want to turn counter clockwise? If so, it is unlikely that a dragging brake is part of the problem. And the CW vs. CCW rotation confirms that differential is sound.

If you jack one wheel off the ground (with the opposite wheel still firmly on the ground) and engage drive, does that wheel rotate easily, or does the engine sound like it is under some load?
Next, if you move the jack to the opposite wheel and repeat the test, does that wheel spin easily, or does it require a lot of accelerator pedal to spin it?
If either wheel requires a lot of accelerator pedal to incite rotation, there may be a problem with the brakes dragging on that wheel.

If, when you jack either drive wheel clear of the ground (with the opposite wheel still on the ground) the wheels spin easily.........the problem is not in the drive axle at all.
If this turns out to be the case, then there is a problem in the frame geometry of the truck, which is causing one of the drive wheels to be "unloaded" when the truck is on level ground.
The reason for this is that the steer axle is mounted to the frame with a flexible/pivoting "trunnion" at the center of the steer axle beam. This allows the truck frame to adjust its angle slightly as ground/pavement/surface conditions and elevations change in order to keep the left and right drive wheels BOTH pressed evenly against the traction surface.
Problems of this nature are worse when the truck has solid rubber tires. Pneumatic tires are more adaptable to uneven traction surfaces. But, on really poor or rough surfaces, just having one of the pneumatic steer tires under-inflated can cause some traction problems too.

I have seen a truck (not Clark brand) where a steer axle king pin bearing failed and ate up the steering knuckle and axle end allowing the left side of the axle to settle much closer to the ground than the opposite side of the steer axle.
The result was that the entire frame of the truck got light on the right hand side.
That truck would spin its right drive tire at the slightest provocation..........until you picked up some weight with the forks.

So, you are going to have to do some careful testing and inspecting, maybe at both ends of the truck to find out why it spins one wheel.

If you have not tried lifting something (heavy) to weight the drive axle (increase traction), you may want to do that too.
If you load the drive axle and traction increases, that pretty much points to a frame geometry issue as the cause.