There are 2 things that could be the problem. Either the bellows has water in it and the shaft is rusted to the gimbal bearing or the worse thing the shaft splines are corroded in the drive coupling.
Wedge a block of wood between the top of the drive and the top of the gimbal ring. With the rams connected have someone run the trim pump and hopefully the drive will back out.
If the shaft splines are corroded in the drive coupling then the shaft has to be cut and the engine removed which will allow the drive to be pulled.