take trans oil filler cap off stick a long strong screwdriver into hole and work shifter rods back and fore.
hopefully the trans shifter will unlock and you will be good to go.
when shifting make a effort to never, never pull up on the long shifter rod when shifting.
always use some down pressure or just side pressure, but it seems like pulling up on the shifter lever when shifting will lock the gears.
shifter rods are worn and you can live with it, if you learn to not pull up on shifter. after you unlock it a few times, old dogs can learn new tricks, and not pull up on lever.
good luck and let us know how it works out for you. when was oil in trans/hyd system changed, may be time for a little maintenance.