so i was driving along the other day when all of a sudden truck stopped running. i look under the hood and see harmonic balancer hanging off belt, this was an already replaced harmonic balancer by the previous owner. local mechanic took a look and told me crankshaft was snapped and engine would have to be replaced. but hoping to get a second opinion on here.

 

I can't tell if the crankshaft is broke, but it sure looks like the bolt that holds the balancer is broke off inside the crankshaft.

There are not enough threads to hold the balancer on, the bolt probably broke on the PO and he put a shorter bolt or used a bunch of washers in that held just long enough to sell it.

From the looks of the remaining threads getting the rest of the bolt out probably wont be easy

 

Looks like the crankshaft is okay, but the bolt that retains the harmonic balancer has snapped off inside the crankshaft. I don't know if it can be extracted, but I would sure try my best. Get a left hand thread drill bit and an extractor bit and see if you can get it to turn out. If so, then it's just a matter of cleaning the threads and putting on a new harmonic balancer with a new bolt.
Those bolts often snap when reused, I believe that particular bold is a torque to yield design.