Yep they do go bad. Sometimes the outer pulley part will just slip a little due to rotted rubber and then your timing marks don't line up. I'd say your mechanic is telling it like it is and you need a new harmonic balancer.
You might try some salvage yards that cater to box delivery trucks. There are bound to be some in the DFW area.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)