have someone in cab and wiggle shift lever while you take a pry bar under cab and above the trans where all the shift linkages are and pry the hi-low linkage back into nuetral. You will still be able to use tractor if you need to if you carefully put lever back into nuetral all the time before going to park or reverse.
Problem is worn linkages and bushings from lever to shift rail, it may need new/rebuilt trans. cover. If your shaft coming out of cover is tight both detent wise and side to side, may just need linkages and bushings along with a little welding on worn shift lever.
Common problem but frustrating
Pull floor, seat and seat floor and you will see where its loose, then pull shifter assembly with cross shafts.