Right, so that rubber thingy is a "Drive Saver", an isolating element between the gearbox
output flange and the prop shaft flange.
We can see the four studs sticking aft from the drive saver, and in between each stud is a counter bored recess which will contain an Allen screw, or perhaps some other type of fastener. The four Allen screws hold the drive saver onto the output flange of the transmission
. The Allen screws thread into the output flange......there are no nuts to counter hold.
Probably your best bet is a strap wrench to hold the drive saver and a breaker bar to loosen the fasteners, or an impact wrench should do the job.