I had that issue for months and it was the toughest to diag. Really frustrating.
Checked the Electrical > Replaced the Speed Sensors > Had the tran fluid level checked (which was goofy cause the car never leaked anything) > checked the Brake Pedal switch.

At that point it was the limp mode that told me it must be the ABS.. they replaced and bled the brake lines. Make certain they bleed/flush them.

*I changed the Speed Sensors first because it happened soon after new brake pads were installed. I never saw any leaks / Brakes were strong -- so I figured the brake lines were ok and no bubbles.
..but after the fix I always wondered if maybe there was air or bubbles in the brake line that tripped the ABS/Limp mode. I'll never know.