We had a 1206 that was doing the same thing and we ended up splitting the tractor and found out that the spacer washers between the gears on the shaft for high, low, and reverse were smashed from someone slamming it from high/low range to reverse I’m assuming. We replaced all the washers and it fixed the problem. Dad says that they are thrust washers and since they were smashed it was letting the gears slide past one reverse and not letting it slide far enough to get into High/Low
