I have the same problem. I've never dislocated my shoulder, but I've been doing the hands-on-keyboard pose 6-8 hours a day for the last 10 years. So, my shoulder flexibility is garbage.
What I do is work it into my warmup routine for the squat:
2 sets of 4-8 shoulder dislocates. My gym has a long wooden staff for these, but a broomstick works fine.
3 sets of 5 squats with just the bar in high-bar position, pinky finger on the marker, but concentrate on pushing the elbows back.
By this point I'm just about warmed up enough to get halfway to low-bar position.
1 set of low weight, maybe 10s, in this halfway to low-bar position.
At this point, I can usually get into proper low-bar position.
This usually sorts me out.
Prior to this, I was only doing high-bar because I couldn't get my shoulders to cooperate. After a week or two of high-bar, I noticed that by the time I got to my work sets, my shoulders were loose enough to get the bar situated properly, so I switched to this routine. I've been doing this for almost 2 weeks now, so I'm hoping in another few weeks I'll be able to go straight to low-bar.