If the cable does not turn inside the cab, then the trouble has to be EITHER
the gear at the transmission end of the cable, or the gear inside the
transmission.

HINT
The gear on the end of the cable can be turned in its mount to accomodate
various gear sizes that might be selected to correct for various tire sizes
and gear rations. If one buys larger tires, the speedo will no longer be
accurate, and one would swap the gear to make the correction. Atlernatively,
if one changed the gear ratio in the differential(s), the speedo would also
be inaccurate, swapping the gear would correct for this. If one swapped the
gears in the diff(s) and bought oversize tires, then swapping the speedo
gear would correct for the resulting error.

When the gear is installed, it MUST be turned to properly mesh with the gear
inside the trans (tcase). If the gear is not turned to properly mesh with
the gear inside the trans, then the gear inside the trans can become damaged
over time.

I suggest loosening the mounting screw and puling the gear and its mounting
assembly out and looking at what is happening inside the trans. Then, put
the gear back in, and twist the assembly to mesh the gears -- the gear you
just looked at and the gear that is inside the trans/tcase. My guess is that
you have no clue that the gear can be changed to correct for error that come
from tires and gear ration changes, and you putthe gear in, but failed to
twist it in the mounting hole to force the gears to mesh.

If you look closely at the case of the transmission or transfer case, you
should notice markings (26, 27, 28, or whatever) that denote the proper
alignment of the various speedo gears that can be used. Count the teeth on
your current speedo gear, and then align the mark on the housing with the
appropriate number on the case of the trans. (If you have a 4WD, the gear
will be found in the transfer case.)