Hello Les,
The solenoids do give problems on these vehicles. They are internaly different.
The last time I bought them you could only buy as a pair.
Have you tried a Mityvac on the Actuator Chamber?
make sure the actuator slides all the way over to engage diff then see if warning lamp goes on/off
Check that the vacuum tank hasn't rusted through.
also there is switch on the diff which tells the system that the front diff is engaged/dis-engaged check that its not seized and works open and closed circuit.
Thanks,
Ian