The item we are talking about is the "PSOM".
A bouncing speedometer is usually a dirty VSS sensor in the rear end, some times you can take it out and wipe the filings off of it but if this is the case you have problems in the rear end. Also the tone ring might not be aligned right.

The speedometer is just a voltmeter and a good indicator of the information coming out of the PSOM.

If it is not in the rear end check the wiring from the rear end to the dash and unplug the ABS module and see if that helps.

If not replace the PSOM and recalibrate it.

You can get one installed by a Ford dealer ($$$) but they will not sell you one.

These PSOMs were upgraded in 1994 but any PSOM should work from 1992-1996 from an F-series, E-series or Bronco.
I have not heard anyone say they have found a PSOM that did not work from the other years yet.
Just get a cluster out of a salvage yard, pull the PSOM out of it and put it in your cluster.
Note that the PSOM that you put in will have the mileage on it that the truck you took it out of had.
Note also the Government frowns on this swapping on the mileage basis and may be illegal.
But a lot go ahead and do it anyway, not that I am telling you to.