Rear brakes lock up?

[White70C20]

I recently had my truck in the mechanic because the rear brakes were hanging up. It turned out the emergency brake had accidentaly been slightly engaged. While it was there, I had him check it over to make sure it would survive a trip to Kansas later this month.

I got it back home today, and while cruising the back roads in our area, I noticed that the rear brakes were locking up (I almost got it going sideways).
The rear brakes had never locked up before I took it to the mechanic. Should I just back off the rear brakes a little bit? Is there something else I should do?