depending on the drive it could ne numerous things. from what i am gathering that age of a sea ray could have a mercruiser R drive or a omc stringer drive. if it's the omc stringer we will just move on and you probably don't want to fix it. if its the r it could be anything from a intermediate shift cable being out of adjustment, or it's outright bad. but if you say it is shifting ok in reverse it's more than likely not a bad cable. what i would do to diagnose is remove the cable from the helm at the engine and try shifting from the engine. this will eliminate the control box and that cable. if it shifts you have a problem with those items. if it still does not shift it is either down in the bellhousing of the outdrive or internal in the drive. does it make any effert to go in to gear like a grinding or clunking sound. you could have a worn shift spool or clutch dog in the lower unit.