All the suggestions are excellent.I have also found the tire pressure on my tractor changes daily.When the differential is excess this changes the circumfrence,causing hard shifting.Something else you might consider,
I fought with my L3400 in the early days but realized if I release the tension on the drive train it doesn't take much force to engage/disengage. Hills and hard surfaces are the toughest for me, since my technique has been to roll forward or back a few feet, a few times.
This sounds like great advice. Like, kick myself for not thinking of it advice:
I have had Kubota 4 wheel drive tractors since 1980. The first 2 were gear drive and this is a HST (B3200). I have found that the tractor will go in to and out of 4 wheel drive very easily if I am rolling on a level surface without pressure on the drive train. Start moving and let off on the forward HST as you push the lever in to or out of 4 wheel drive (do not depress the clutch). It falls in and out very easily. It will be difficult if there is any strain on the drive train.
I hope this helps.
I have a B7500 and it slips in and out smooth as butter, and I shift it often, the only thing i do is make sure i am moving straight ahead for about 5ft before I try shifting it, i never have any bind up.