Abs light is working when I start my Jeep.
Light goes off and everything seems to be ok.

But rear brakes locked! and when I brake heavily there is no that normal ABS "grumble"

Any ideas?

 

What is the vehicle year?

 

If it's the 97 ZJ in your signature, you may only have 2 wheel ABS, which only works on the front. It's still unusual because most of the braking effort is done by the front wheels anyhow. Maybe your proportioning valve is stuck. When I first bought my WJ, it had been sitting for awhile and it seemed like it took forever to stop. My mechanic and I bled the brakes and I wasn't getting any pressure to the front calipers. He had to open the bleeders on all 4 wheels to get fluid flowing again. They've worked well ever since. I also had a vacuum whistle from the power brake booster from under the dash under heavy braking. That seems to have cured itself too.

 

Yes.. 1997 ZJ
I think this is "full" 4 wheels ABS.
"proportioning valve is stuck." How I can test that?

 

Try bleeding the front brakes. If you don't get any fluid out, open all 4 bleeders and hit the brakes. The pressure drop should reset it.