Using your meter on the output pins of the chip tests its output transistors. While this can identify faults with the chip, good readings don't tell you what's happening upstream within the chip.

Since both the 4Q and /4Q outputs are struggling to change, it says the predrive transistor circuit in the chip for flip-flop 4 is likely suspect.

In your shoes I would still replace the chip.

74175_outputs.png

Great, and thanks for the update!