97 Tahoe and 99 Tahoe different between the ecm

Well, it's actually more important that it match your engine type than year.

Even though sensor types have been known to vary from year to year; you should try to get one matching your year. They absolutely vary from engine model to engine model. An ECM from a Diesel vs Gasoline engine is a night and day difference.

There are two parts to a engine computer. The hardware (pcb and its components) and the software (programming of how it functions). In most cases if the hardware is the same then you can re-flash the proper software and there will be no issue's. In the case of GM ecu's you only need to worry about matching the service number. In some cases there may be different hardware versions that can handle the same software but matching to your original is just a easier rule of thumb to follow if your not familiar with the units.

In the example below you can see the "serv. no." This is basically the hardware version of the ecm. It can handle software for a lot of different year,models and engine types. The lower underlined number is the software version which can be re-flashed and is more specific controls for what it comes out of.