Thanks for the replies. I will start with opening up the top as suggested, and see if I can identify the defective part/linkage/etc. If it's anything serious, I will just continue using it in low while I look for parts or complete replacement. This lift really doesn't see enough action to make much of a difference - I just prefer to have everything functioning as intended.

Does anyone know who might have made this transmission for AC? It would be much easier to troll the internet for parts/replacements if I had a manufacturer/model. AC searches are coming up empty for this specific setup.