It's the little motor inside the cluster for the tach.
They usually don't die, so one from a wrecker cluster will work.
Are you considering changing the dash to the '95 up?
People have done that, but IIRC, it wasn't a simple swap.
1995 is the odd-ball of the these trucks, so it may be best to look at '96-'98 as a donor. Not sure if the '95 has it's own issues when it comes to how the electronics in the dash. Those '95's are not true OBDI nor OBDII. Sometimes referred to as OBDI.5.
Ditto for the steering column. It may or may not be a prob., but why find out if it's not going to work via trial-&-error?
As far as the seats, later model ('95+) extra-cab passenger seats require some mods to either the floor or the tracks/risers to be able to fit a Standard Cab. Other than that, virtually any seat from a GMT400 will work.
There are members here that have utilized seats from other vehicles, such as AstroVan & Pontiac Bonneville seats.
I would recommend sticking to GM for alternate seating. GM was pretty good at retaining the bolt patterns so that you can re-use GMT400 tracks & risers.