We have one bathroom on each floor, both with kohler one piece low volume flush toilets; about 2 weeks ago the second floor started to not flush properly; it fills up like it is clogged and will overflow but then it slowly drains. I have tried a plugger, a short wire snake a and I even crawled onto the roof to check for a blockage in the vent pipe; nothing has worked so far. I am about ready to pull the toilet off the floor, There is no problem with any other items. i.e. sinks, showers, washing machine, etc. Any help?

Pulling the toilet and power snaking is your best option.

I have been on a service call before and sounds like the same situation and after I pulled up the toilet I found something stuck in the p trap in the toilet start there
Ps you'll be surprised what you might find

GOT kids ? If so , Ohhshhht is right If not he still be right.

Toilets do not have P traps but there is an integral trap which can be rodded but not by a short wire. You need to rod it with a toilet auger.http://www.youtube.com/watch?v=4oyfjKLXwHk If that does not work you then rod the roof vent stack with a snake long enough to reach roof top to basement plus 20', visually inspecting the stack does not work.