I use a 12 ton hydraulic jack and a variety of wooden blocking. The blocking are ends from a timber frame barn and re roughly 10 inches wide by 3 to 4 feet long in a couple of depths. I also have a pile of short 2 by 8s and some plywood scraps. Jack it up to the maximum jack height and then block up under the axle, then drop the jack so the axle is supported by blocking and then retract the jack and put a block under it and jack up again. Slow but safe.
Note that the SEEs were supplied with a 10 ton long stroke hydraulic jack but its pretty rare to get them when they go up for auction. I don't know where they go but rarely see them for sale. There is at least one firm in England that sells Unimog parts on Ebay that will ship to the US who sells new ones. You want the 10 ton version.
Make sure whatever you use to jack it up that you have either an appropriately rated jack stand or blocking as a backup. Be aware that the weight of the SEE concentrated on the base of jack will sink into the ground unless you spread out the load. My cheap jack stands will sink into asphalt paving unless I put a sheet of plywood under them.
Make sure you break the lug nuts loose before jacking. Unless you have a suitable breaker bar, plan on using a long cheater pipe. Don't jack too high, just clear the ground as the weight of the tire and rim is more than most folks can lift. just clear the ground and then rock it back and forth until it comes off the studs. I use some thin scraps of plywood to roll the new tire in place so that the lug lines up with a hole in the rim with the tire at slight angle, once I get the rim on the lower lug I then can push the top of the tire so its vertical and it levers onto the other lugs.
One last thing, if you are messing with the spare, spray the hold down bolt down with penetrant a day before. Its a fine thread and inevitably is either rusted in place or really hard to remove. When you are done definitely put some grease on it
Its very dangerous to change a tire with just a jack holding up a vehicle, many folks have been seriously injured or killed by a vehicle falling over on them. Note that the tire and rim is quite heavy and an average person isn't going to be able to lift it in place.
While you have the tire off and the SEE supported, good time to grease the fittings and check the portal fluid. The Unimog expert I bought my 1300 from suggested squirting a bit of penetrant into the disc brake bleeder screws to keep them from rusting in. You can do all these things with the tires on but easier with them off.
