Tossing this out for discussion. I had not seen any of these before except for cars.
I'm pretty sure that I need one.  
I believe I might want to make it a bit beefier since a number of my tractors are a little heavy like my Farmall Super M, the Super MTA, the 400-LP and the MF-165. I also believe that I would want heavier casters.
Since my shop floor slopes a little excessively (bad decision at the time of pouring) I would want very positive locking casters.
As you can see it is adjustable in length and you can also adjust the width of the rear axle supports. Just looks like a good idea. It also appears that it is easy to break-down for storage.
I'm on a lot of tractor sites and I am always amazed at how many tractors I see being worked on sitting on concrete blocks turned the wrong way (as if there is a right way :eek:).

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That is a pretty interesting tool.  I can see where that would be a good thing.  Especially for some of the lighter tractors.

Ron

Looks like he used the online plans for a rotisserie and modified the uprights. Not hard to make as long as you can afford the steel. I'd use 4" square tube, 3/8" wall for the end supports. Heavier casters, or if you don't plan on moving it around while you work on it, use some that can be retracted up and set the support frame on the floor.