I have a Montana R2844 manual shift. my tractor did this, follow your range shaft to where it meets the transmission, see if has a roll pin in it, if it is backed out tap it back in. this fixed mine for awhile but know its doing it again. I took the roll pin out tonight, thinking it might be weak and bending a little while trying to get it into high range. going to put a new one in it in the next few day and see if it solves the problem. If not I got a buddy mechanic that says he will come take a look at it.