1988-1994 is the same. There are 2 clusters. One for manual transmission and one for automatic. dakota digital makes a direct bolt in new for $850.
Why would you say 1988-1994 are the same when they're not at all?
1988-1990 have a couple varieties of the "moonie" gauges
1991 has both moonies and needle style but the electronics are similar to 1988-1990
1992-1994 has a couple varieties of needle gauges (tach, non-tach) and the electronics are totally different from previous years, connector is different, even the dashboard is different where the connector attaches.
Then also don't forget some are gas and some are diesel, etc. etc.
I'm sure OP is asking about changing from moonies to needle gauges. "Easiest" is finding a 1991 needle cluster; all that is needed is to move 3 wires around in the harness connector as the pinout is slightly different, then calibrate the speedometer by either using a GM programming kit (essentially jumpers) or modifying the cluster with a DIP switch. The hard part here is finding the somewhat not-common 1991 needle cluster.
Next up is swapping to a 1992-1994 cluster which requires a bit more work. Wiring in the DRAC module (and modifying it with a DIP switch to calibrate the speedometer), changing the harness connector, and making a little pedestal in the dash shell for the connector to sit so it won't pop out of the back of the cluster would be smart, too.
There's write-ups out there detailing this swap. It's a bit of work. Or yes, you can buy an aftermarket cluster ($$$$) just be sure you're getting one specified for your year model, and don't forget you may need to buy a number of new sensors/add-ons to be sure every function works - their site should give all this info.
Richard