Kubota R510 Wheel Loader + Cab and backhoe, JD 6200 Open Station, Cushman 6150, 4x4, ten foot 56 hp Kubota diesel hydraulic wing mower, Steiner 430 Diesel Max, Kawasaki Diesel Mule, JD 4x2 Electric Gator
Creeps meaning it moves slowly when it should stay parked or moves very slowly when it should move? Usually when a hydro creeps in the former meaning, it will do this in one direction only as the swash plate is not adjusted to stay in the centered position.
At once? I'm kidding with you. Grease fitting only will/might help if it is sticking in forward or reverse. Likely will need adjusted slightly. My eMax has a turnbuckle adjustment to center the control under the deck.
www.jdparts.com for parts manual might help you locate stuff. but a service/shop manual would more clue you in on what you need to do more exactly.
looking at... TractorData.com John Deere 855 tractor information it looks like your tractor model has been made for many... many years. might suggest being careful and double check what manual you look at.