They are locking up in gravel after barely touching the brake pedal.Adjusters are backed off.Any ideas?

Check the orientation of the brake shoes per wheel;
Both brake-shoes have different sized/length brake-lining.
The shoes with the shortest lining should be mounted towards the front of the car.
The ones with the long lining towards the rear.

Also,
file or grind some bevels on the leading edges of the brake shoes if they don't have that.
This prevents 'grabbing' of the brake-shoes.

Brakes are like a mullet short in front and long in back.

Is this after the car sits awhile? If so it is "somewhat" normal. If there has been a master cylinder change somewhere along the line you may have a master with a 1" or less bore which is a manual set up. Best of luck to you.

Clean and grease the contact points between the shoes and the backing plate. Best tell tale sign is if they grab hard first few times then settle down. With out the grease they get a little stuck and when you apply enough pressure to unstick them it's enough to lock them up.

Why are the adjusters backed off?

When you put the drums on, there should be some drag. If the shoes are pulled away from the drum because the adjuster is backed off, the lockup is happening because the slack is being taken up suddenly. (I can picture it better than explain it).

Have the shoes ever been correctly adjusted?

Sounds like maybe a leaking axle seal.

or a wheel cylinder

back in the 80s I had a 76 Charger and a 75 Cordoba. Dad had a 78 Monaco wagon. seems like all of the "late-B" bodies were all like this. My current 78 Sport Fury is the same way. Nature of the beast. I remember seeing a tsb somewhere about this, where they said to put smaller wheel cylinders on them for the issue.
All the others I mention had had their rear brake shoes replaced during our ownership. My 78 still has its original everything on the back. (only 54k miles)

I also remember in my work, seeing a tsb (yeah I know, not a Mopar) about Rangers, Aeroflop vans from the late 80s/early 90s and I don't remember the complaint (besides being Fords) but the tsb said to replace the shoes and they gave a pn for the updated shoes..... IDK what you have on tehre for brake shoes, but unless you have some OLD stock on there, its hard telling what the composition of the brake linings you have may be.....

Thanks everyone.Brake shoes are on right.no leaks.new master cylinder.lines are hooked up correctly.

for sure post what it ends up being.

You didn't answer my questions.

Yes.They are adjusted to a slight drag when removing brake drum.I backed the adjusters off to try it.No change.

if the emergency brake is on just a tad they will do that

Watching.

I've had probably 2 dozen Magnums and a few Dobas, NEVER had that issue. I"ve had master cyl's go bad and not release the brakes till I loosened the line at the MC

Could possibly be the rubber hose going to the rear brake lines internally collapsing, therefore not allowing the hydraulic pressure to release. I had this happen on a Dodge Durango. If you drove long enough the rear brakes would drag on the drums and heat up.

Drums don't heat up with driving

they do if the shoes are dragging

Shoes aren't dragging.Drove it 2 hrs home.Brakes are hyper sensitive in gravel and lock up.fine on pavement with no pulling or smell as if it were hot