From the shop manual:

SRS warning lamp
The SRS warning lamp illuminates for 5 seconds during system readiness check on starting the engine. The SRS warning lamp extinguishes for one second after the system readiness check is performed. If a fault is present the warning lamp then illuminates continuously. If no fault is present, the SRS warning lamp remains extinguished. If a system fault occurs in excess of two seconds after the readiness check has been completed, the SRS warning lamp
illuminates for that ignition cycle. It remains illuminated for a minimum of 12 seconds ± 4 seconds for all fault conditions.

The SRS warning lamp will only illuminate for low voltage concerns while the low voltage condition is present. If the low voltage condition is corrected, the SRS warning lamp extinguishes and the fault is recorded in the DCU's memory.

Where are you getting these alternators from?

 

Besides low voltage there are 2 other common reasons for the SRS light to come on on a DII.
1-the passenger seat belt buckle is dirty from lack of use, buckle and unbuckle the passenger seat belt a dozen times or so.
2-the wire harness at the seat belt pretensioner(s) gets knocked loose/dirty and needs to be cleaned with electrical contact cleaner or just unplugged and plugged back in.

 

I got my alternators (except one)from O'reily's auto parts. I was driving home one day and my trucks rpm gage stopped working. Plus the lights flickered. Then it dropped dead/stopped running. I thought it was the battery so I replaced it. Then I got an alternator from the junk yard which worked for about a mile or two and quit.I went to O'reilly's and got another that worked not even 5 minutes after installing the rpm gage stopped and so did the vehicle. Went back got another put it in and none of the gages worked. So put another one in and it worked fine drove it about 400 miles then noticed the lights flickering again. At one point the rpm gage stopped working again. So I'm wondering if there is an underlying problem that I am missing.

 

As bad as the rebuilt alternators are from the discount auto parts stores I doubt that many went bad.
You have a electrical problem somewhere, double check all of your connections on the back of the alternator or where it grounds to the truck.

Also double check your belt routing and belt tension.