If you aren't serving that 88mph with a fair amount of spin, then, yeah, you aren't hitting a very big serve.
Good form and a small change in timing should allow you to push it up over 100mph assuming no physical injuries or old age.
I think it is worthwhile to work on more pace as long as you warm-up first and keep the numbers hit reasonable. Also, do exercises to protect the shoulder. If things click an extra 15mph might not be too hard to find. Good luck.