Welcome to the forum.
Has it been determined that the cable is the problem source? Has the cable been detached and the shift mechanism been manually actuated to confirm proper operation? With the cable detached has difficult operation been confirmed?
Not at my 2000 380DA to confirm lengths, the parts manual says longer is on the starboard side, I would think longer is on the port.
974170 PORT SHIFT CABLE, CONTROL TELEFLEX TFXTREME 21'
974162 STARBOARD SHIFT CABLE, CONTROL TELEFLEX TFXTREME 24'
Neither cable is available on the NOS Sea Ray parts website, FP Marine.
https://www.fpmarine.com/default.asp
On the plus side, ALL of the marine parts websites sell custom length Teleflex cables.
On another plus side fortunately the 380DA has excellent access for changing that part. Put a folded towel on the starboard helm shelf, unscrew the helm switch panel and flop it onto the towel, unscrew and remove the starboard helm storage, knee level, to provide complete access.
Please report on results.