I don't have any experience with signature analyzers. If all the ohms ranges are functioning, you can probably rule out the communication between U462 and U102.
The only other thing I would look at is K101. It's default state is closed. This would bypass any of the attenuation if it did not open for the higher voltage ranges. I have not opened my 3478a in a while, but if you can verify that 5v is measured across K101 and 0v across K102 while manually changing the DCV ranges, then it is indeed looking like U102 has failed. I think you may be able to reseat U102 based on some of the pictures I have seen.
Also, the 3468A repair manual may have additional info even though they use different U102 parts.