My 2007 T300 lunges in gears 6-3 and 6-4

I have a 2007 T300 transport unit that I use to haul my JD J450 tractor. I've noticed the last few times (3?) that I've driven it that the truck lunges, almost like it has a stuck brake shoe, in gears 6-3 and 6-4. It totally smooths out in 6-5 and 6-6. This is an extremely low mileage vehicle, at 7767, as we use it typically for fire response and occasional plow jobs. Any insights?