One way to narrow this down would be to test GPIB independently of VISA.
You can try running "ibic" (usually in /usr/bin or /usr/gpib, maybe). This gives you a command-line interface to send NI-488.2 commands.
In ibic, you could try something like "ibfind gpib0" and then "ibfind dev10" (or whatever GPIB address you are using for an instrument). If you've gotten this far without an error, your GPIB driver is probably okay. (To be sure, you could 'ibwrt "*IDN?"' and "ibrd 500" to query the instrument.)
Next, you could try the VISA utilities for Solaris. These are installed by default in /opt/vxipnp/sun/NIvisa. Try running "NIvisaic", which is an X Windows application to let you see devices and send commands.
Note that if you haven't installed VISA
in /opt/vxipnp, you need to set the VXIPNPPATH environment variable for LabVIEW to be able to find it.
This will help you narrow it down to see whether it's a NI-488.2 problem, a VISA problem, or a LabVIEW problem. Report back here with your results. I hope this helps.