right rear wheel locks up after short drive releases after 10- 20 minutes

Are you sure you don't have a brake shoe that came off the steel backing and is caught up in the rear drum?

mechanic says brakes are fine

the bearings in the wheels might be bad and might get cought on somethig?

Sounds like the pressure is no releasing on your wheel cylinder. When it locks up crack open line or beeder valve on that wheels wheel cylinder. If after doing so it becomes unlocked I would replace wheel cylinder and also check for any possible blockage in that brake line.

I have heard of this happening one other time, but it was on a motorcycle. If I'm right the solution would be to flush your brake fluid. Brake fluid can absorb moisture. If it absorbs enough moisture, it can cause this problem. Basically what happens is the water expands in volume significantly as the vehicle heats up. This expansion inside the brake line increases pressure on the slave cylinder, and applies the brakes. When the vehicle is allowed to cool, the water will compress back down and release the brakes. I don't know for sure if that's your issue, but it might be worth checking out, especially if your brake fluid is old anyway.

sounds like a smashed or crimped brake line to me , brakes heat up , expanding fluid sets brakes even harder. after cooling fluid has no pressure in the line until brakes are used and it starts all over again ...

kyle's answer is possible and with rust being an instant by product to sieze a wheel cyl. YES I agree

It is true , the rear diff does lock up without warning. It's not about the brake linings or bearings