After squats my low back tends to be tight and a bit abused due to the 400lbs it just endured for some time.

I find a good trick to sort it out and relax the lower back while getting a good stretch is to use the hamstring curl machine next, or just after doing glute ham raises (which will further tighten your already abused lower back and core).

The heavier the weight you can ham curl, the more of a stretch and relaxing feeling you will get in your lower back, not to mention a productive burn and hypertrophy to continue blasting those legs.

Try it and see how it goes for you.