Have a look on top of the g/box where the selector cables attach, in that housing is a shaft going through an oil seal, under that seal is a needle roller bearing which are bad for seizing, a sure sign is a brown residue round about the seal itself,give the shaft a good lubricate with spray grease/oil and get some lubricant down into the bearing, if that is not your problem then that housing may need to come off for a look internally,good luck.
cdc, March 2016