I'd really like to avoid rerunning the line if at all possible. What are my options?

Thanks

I use a short appliance cord and a single receptacle under the sink.

Not an NEC requirement.

Waste grinders are 'permitted', not required, to be cord-and-plug connected. See Art 422.16(B)(1).

And I didn't say it was 'required' to be cord and plug connected. That is why I specifically stated "....or other flexible means...".

I also assume you have never wired a residential disposal before. The wiring compartment in them is painfully small. I wouldn't even want to think about wiring one with 14/2NM let alone 12/2.

I am well aware of 422.16(B)(1) thank you. I have wired many of these in residential kitchens.
Larger commercial units are always hard wired, at least in my world, but I use sealtite, not cable.

That may be your interpretation, but many AHJs have no objection to hard wiring a GD with NM.

It is NOT a wet location and the device is rigidly mounted.

The not following the contour of the structure is pretty hard to get around.
The physical damage part is pretty sketchy as well. Under my sink sees quite a bit of action.
I hope I did not imply that I thought it was a wet location. It is obviously not, as you say.

In all my years I have never seen a disp hard wired with NM.

However, two of them are in a duplex I own, which was built in 1979. The original owner of this duplex also owned one of the area's big electrical contractors, for what that's worth. It struck me as a bit odd, but I've also seen water heaters and furnaces wired with unsupported, unprotected NM - AFAIK these were pro jobs too.

It's not really a hazard and there are more important electrical issues to really get concerned about and more important battles to fight....

I've used flat HD 3-wire appliance cord in the past, looped up high, but may go liquidtite in the future, esp. in rental props... belt+suspenders or OCD?... you decide. ;')

Wet locations are deinfined in the NEC, and under a kitchen sink is NOT a wet location.

Wet locations are tub/shower areas, outdoors exposed to weather (but if some cover is provided it may be a dmap location).

The NEC is not designed to protect the wiring in the event of other failures,like a leaking drain.

There is not enough vibration form a correctly mounted GD to bother much of anything.

IOf the drain lines are coming lose the GD is not correctly attached to the sink flange or the lines are not correctly tightened.

It takes a pretty decent force to tighten the locking collar of a GD to the flange (for many units the 'jamb wrench' provides the necessary leverage to get the collar fully tightened.

If you have enough water under a sink to cause an issue with a GD, you have probably wrecked the cabinet floor also.

What is going to damage NM under a sink?

Brickeyee's reply is a perfect explanation.

I am not sure what you trade is but I suggest you read up a bit on the electric code before making such assumptions or conclusions.

e.g. a kitchen GD has an upper, or "wet" compartment, where food waste and water churn vigorously, and a lower "dry" compartment where the motor and wiring live. However, the motor shaft passes through both compartments, the water-tight seal provided by (IIRC) some combo of bronze bushings and rubber O-rings... sounds like the OP suffered a failure in this shaft seal, and water found its way into the NM-B jacket. It's a cautionary tale which makes me question the "best practices" for wiring up a GD, is all I'm saying.

What is going to damage NM under a sink?

I dunno... Drain-o Max Gel? Metal buckets filled with metal cleaning utensils, given a hefty "thrust" from a slamming cab door? Toddlers w/ silverware? Rodents?... in general, "ig-nernt" tenants... or their ig-nernt guests. Admittedly, NM-B is tougher than it looks, probably way tougher than appliance cord, but certainly less tough than MC or LTite.

There is not enough vibration form a correctly mounted GD to bother much of anything.

LOL... the world is full of UNwise peeps (and mechanically UNsavvy HOT girlz)... and thin gauge stainless sinks, stamped out in Cheena... when said peeps shove a MANGO PIT(!) down a GD mounted to said ChiCom sink, HMOG, the WHOLE COUNTERTOP "DANCES", Mr. Bill! =:O

The NEC is not designed to protect the wiring in the event of other failures,like a leaking drain.

Well, the NEC is from Man, not God. Obviously Duh Code has shortcomings, hence the tri-annual updates. ;') Of late Duh Code seems blithely unconcerned with undersink plumbing mayhem, yet overly concerned with the hypothetical tangling of appliance cords on the counter tops above.

After typing this up, I conclude that while NM-B is perhaps adequate for the GD hookup, probably the more important practice is regardless whether cord, conduit, armored, or NM-B is used, this final "whip" should route immediately UPwards, and/or have a "drip loop" formed, so that leaks internal to GD don't propagate water back to j-box... agreed?

PS: It's not clear whether OP has proper j-box under sink, but it sounds like he's screwed, i.e. needs to replace a goodly length of his NM-B, eh?

I'll fix this up right quick, gooder'n the old one, with nothin' more'n a single-gang "old work" box, a blank metal cover plate with a knockout, some FNC, a couple clamps, a couple'a wirenuts, and a bit of THWN.

If y'all can't figure out what to do with that parts list, I give up on yas.

That said, I think a liquid-tight whip is probably overkill.

Someone will chime in that this met code (in Podunk, 1897), but I won't rent out such a prop until corrected, w/ FLEX/FNC clamped to wall as far as possible.