I have a leak coming from my shower stall drain. It is a fiberglass bath tub/shower in a 25year old mobile home It appears that as the water drains and backs up, it over flows over the black pipe and onto the sub floor under the stall/tub. Naturally it waited until I was finished putting in a new Pergo Floor. Is there anyway to remove the tub drain and flange from the top so that I can seal off the leak or insert a doughnut seal??

You haven't told us what fitting's leaking.
it over flows over the black pipe and onto the sub floor under the stall/tub. What overflows? You haven't furnished enough information. Can you supply more details? Tom