What would cause rear tires to periodically lock up On a four wheel drive

Wheel cylinder leaking brake fluid, axle seal leaking rear end grease or brakes worn out.

i agree with tuff4x4, most likely your issue, could also be broken hardware (springs) inside brake drum. either way, you will need new brakes if wheel cyl or axle seal leaks because it covers the brake shoes is grease or brake fluid.

if all that checks out inspect the ring and pinion...