I'd just like to give an update to my problem as I've finally able to install 2.2 onto my system. I had tinkered with the SCSI drives, cables, assortment of system tunables, boot loader configs, etc but nothing helped. I was able to reinstall 2.1.5 just fine, but somehow when it comes to 2.2 the system would get stuck at that dreaded "bsdlabel" command.
Then one fine morning when the system booted up with the 2.2. Install CD, I noticed something odd with how 2.2 reported the transfer rates of the two SCSI disks attached to the Adaptec controller. It reported the SCSI disks as having a transfer rate of about 11MB/s. I can't remember how it was reported under 2.1.5, but I have a SCSI-2 disk there, which should give me a transfer rate of 20MB/s. But I also knew pfSense loaded the correct disk driver (ahc) for it. And so, I went to the SCSI BIOS and had a look at what I could change so that pfSense 'detects' the appropriate transfer rate.
Under the Adaptec SCSI BIOS, the "Initiate Wide Negotiation" was set at a default of "Yes" and at the default transfer rate of 40MB/s. I disabled the setting by selecting "No" and with that the maximum default transfer rate went down to 20MB/s (the 40MB/s option was no longer there).
With the above setting in the SCSI BIOS, the installation was no longer stuck at bsdlabel (in fact, the command just did it's thing within a second as it should) and I was able to complete the installation of 2.2 and restored the config.
Hope this help someone with a similar setup and issue.