Well, splunkforwarder-4.3.1-119532-solaris-8-sparc.pkg.Z is a package
and splunkforwarder-4.3.1-119532-SunOS-sparc.tar.Z is a tarball
You can choose either one, as long as you're on Sparc. If you're on Intel, pick one of the other. I agree that the naming of the package as 'solaris-8' is a bit confusing, but I guess that it only wants to show that it's the correct package for Solaris 8 or newer.
I haven't used the tarball, but I assume that it's more along the lines of;
tar -zxvf splunkforwarder-4.3.1-119532-SunOS-sparc.tar.Z && make && make install
Ask your Solaris admin for the correct syntax. Please note that the 'splunk' account in the OS does not get created automatically when installing from a tarball.