This is Guardians of the Flame by Rosenberg. I loved this as a teen as well.
From the book:
James Michael Finnegan/Ahira Bandylegs -- computer sciences major/dwarf warrior
And when you've spent your whole life with muscular dystrophy, you're really lucky, in a way. There's lots more things to dream about. Like being able to punch a computer keyboard at better than a scorching ten words per minute. Like sleeping in a top bunk. Like feeding yourself quickly, wolfing down food so you could run off somewhere. Like using a goddam bathroom without having someone else wipe you off.
Like not having to be so goddam cheerful all the time since because you're a feeb in a chair, people will let you get away with anything as long as you don't touch them.
As regard their arrival and the wizard accidentally blasting things:
The volley of sound flowed into Aristobulus, his breathing becoming deeper, a tinge of pink replacing the fishbelly pallor of his face. The fingers of an outflung arm twitched, then curled, as his eyes snapped open.
Aristobulus sucked in air with a desperate gasp, and a stream of sound issued from his mouth, obscenely guttural and harsh.
And like a striking snake, a bolt of lightning crackled from the tips of his fingers, shattering the nearest of the boxes into a thousand charred, smoking pieces.