Based on where you are descringin the tingling, it could also be an impinged shoulder, especially where those backpack strips lay on your shoulder.

The suprascapular nerve could be impinged at the scaular notch. If it is it could be swollen and pressure from a backpack or mountain biking or even sleeping could aggravate. The fact that you describe relief when the pressure is relieved does sound like a pinched nerve.

Most orthos have a hard time finding this. They just dont ususally think to look at ths nerve first. Seek out a physiatrist and see if they can do an ultrasound of your shoulder. Believe it or not ultrasound is a new technology for the shoulder. MRI's cannot really catch a pinched nerve. Thats why they do a corresponding EMG which would show which nerve is potentially injured.