What's going on with my tansmission?

I have a 2012 4cyl 5sp MT. Sometimes, but not all the time, the transmission is locked up in reverse when I first start the truck. This is usually on a very slight incline. Almost level, but not quite. I can then put the transmission in any forward gear with the clutch engaged, the truck rolls freely backwards. No binding in driveline or brakes with clutch engaged. I can change gears from reverse to first, but when going back to reverse it is still locked up. When the transmission is in reverse and the clutch is engaged, it will not roll. If I actually drive forward, then go to reverse it's not an issue.

Reverse requires a little effort to engage with a small pop, then works fine. Shifts fine after that. From any gear to any gear, no issues. It has never happened while driving and needing to use reverse. Only when it has been parked for half a day or more. And only if I need to reverse initially. Not all the time either. I'd say it will do it about 20% of the time.

Also, when I leave the truck, it is always in first with the E brake on. It shifts from first to reverse easily when I start it back up, even when the transmission is locked up, and again, will roll backward on it's own with clutch engaged in forward gears, so no issue with brake.

 

E Brake is not releasing properly, it's either sticking or the shoes are stuck to the drum until you try to move and cause them to release.

 

So you think it's the brake even though it will roll backward or forward freely if it's in any forward gear or N? How is it engaging the brake at the point of the transmission being put into R?? The pop to get it to engage is clearly not coming from the back of the truck and is in the transmission.