My Pontiac 6000 did the same thing.  Locked up prematurely on the right
(rear) side.  Turned out that both left and right wheel cylinders were
leaking, and they pretty much had to do a whole new brake job in the rear.
The cost to do that and a transmission fluid change was $300.  They said I
was lucky that I didn't lose my brakes completely, so I'd recommend fixing
it ASAP, and yes it is probably dangerous.

Possible wheel cylinder leaking fluid onto the shoes.  This makes them grab.

wife said her car's "acting up"!...

  I haven't checked the shoes yet (it's -15* today) but took it out
for a drive.   When I apply the brakes the rear, right wheel seems to
lock up, even on dry asphalt, prematurely under light brake pressure.
The other three wheels seem fine.  Also, when the car does this the
rear, right end of the vehicle seems to 'dip' down like someone is
pushing down hard on the trunk.

I thought it might be an ABS problem so I pulled the three ABS fuses
under the hood.   Made no difference.  The problem is very annoying
(not to say potentially dangerous) to say the least.

Any ideas?

PS  Yes,  I do intend to take a closer look when I can get the veh
inside.

-Karl