It sounds like one of the geared motors that has the large diameter bearing on one side that the wires pass thru instead of the axle? If so, you might need to get an impact driver... If that doesn't work then it's drill off the bolts heads time.
As a guy who has a shop I get that they just made this a money losing job... But is the shop that did it not stepping up to fix it? Not that you shouldn't have double checked the axle nuts, but a shop shouldn't be counting on you to do that either.
PS... Impact drivers hammer the bit as its twisting to keep it set in the screw head.. they make manual ones you hit with a hammer, and electric ones that look like an electric drill ( but work different inside)