If the lever is completely frozen, I would try some penetrating oil like pb blaster or liquid wrench where the shaft goes through the side of the tranny. That's if the shaft won't rock back and forth slightly just by bumping the handle.
Mine can be finicky sometimes switching between high and low range. Sometimes to get it to pop out of high, I have to let off the clutch slightly to get the transmission just right. Same thing with shifting into low range. I found it goes out of high range easier if the tractor is still rolling slightly. Then just as it comes to a stop, I pop it into low range
