Up sh*t creek...

Well if they have been doing a LOT of sewer work, they MAY have done something so that it now flows slower out of your house. Call them have them look at it. Try to be there when they come by. If you are home and its nothing that they did wrong, maybe ask them if they have a snake to borrow? Or ask one of the workers if he would drop by after hours and do it??We lent them out all the time to the residents.
Ohhh kinda outside the box here, but if you can get them to snake it and they are going to charge you, just ask them if they could put it on your tax bill??

Good luck.

some good info coming in here now. don`t give up, break the plug out of the floor drain, just try not to mess the threads up too bad. you can always get a pvc plug for it. been plumbing for 43 yrs, never have seen a floor drain that wasn`t supposed to be hooked to the drain lines.have done modular comm buildings (for 15 of those yrs.)that went all over the u.s. , including mich., and all of them were to pass code in each state, all f.d.`s were hooked up to the drain system. grandpa may have been a jack of all trades, most were, and pretty smart too, but he may have not been a plumber. not bad mouthing him here! I have really ran in to some screwy stuff done by some people doing there own plbg. some of it even worked, it`s not rocket science by any means, or i wouldn`t be doing it. and by the way, they used to kick kitchen sink drains out of the house seperately then tie them in to the sewer yard line, possibly he might have done that--------bob