wtf? heck no. first off they are like 14.95 each for cheapo replacements. second off they take about literally 20 seconds a side to fix. just take the nuts off and put the new one back on. third off, Thats just there to stop wheel hop it has no bearing on how the car sits...like at all. I'm pretty sure they are blown, every enthusiast driven mustang i've seen has the quad shock blown. They're built like crap unless you get those high performance ones.
take them both off and see if the car sits level, then you'll know if thats the problem. the car doesn't need them to be drivable..then find a new shop
if you look at a suspension diagram, it keeps the axle from moving too far forward on one side, or too far back. IE right side moving forward, left side moving back. It also, along with the control arms, provides a source of stability/alignment for the rear axle.