The hamstrings are a two joint muscle, meaning that they cross the hip joint as well as the knee joint. So it helps to do different stretches to isolate:
- the upper part of the hamstrings (nearest the ischial tuberosity or buttocks insertion)
- the lower part (closest to the knee joint)
Also, the hamstrings are made up of 3 different muscles, basically:
- the mid-thigh portion
- the medial hamstrings (inner posterior thigh)
- the lateral hamstrings (outer posterior thigh)
This video targets stretching each of the above. Notice that he engages his abs to control the spine. He uses his breath, rhythmic, dynamic stretches. Here is another variation with dynamic movements in standing using different planes of motion. See if these help, especially isolating and stretching the upper hamstring.
Also there are the prolonged static stretches as well. This static stretch may be easier for you to isolate your hamstrings from your back than the one in your link. Warm up the muscles first with exercise or heat because you get a better stretch with a warm muscle.
Given that you are so restricted that you can only lift your leg 2' you probably have a neural component to the tightness, meaning that it is not just that the muscle is physically shortened, but that the nerves are also “tense” causing the muscle to “hold” that tight pattern. If that is the case, using PNF - contract relax and/or rhythmic stretches may help you release better.
There are also some physical therapy techniques that you could learn. One is called PRRT that releases reflexes and can be very effective. (Sorry there are no youtube videos. You’ll need to take a partner with you and ask your therapist.)
With your description you may also want to stretch your glutes and hip rotators.
In addition to stretching, you can use a foam roller to self-massage and release the hamstrings.
For additional information on stretching see Daily stretch routine to increase flexibilty and overall fitness? The main thing is to find the stretches that are most effective for you and get a regular routine. Good luck.