Do you neeed to hold a gearbox output or differential input flange to R&R the big retaining nut? On the right is the special factory tool you don't need. Below is a cheap tool you can make yourself.


18 inches is a good length for the straps (or maybe a couple inches less to fit in your tool drawer), and 1/8" thick x 1-1/4" wide. Put the tail ends together and drill one 11/32" hole through both straps for a single 5/16" bolt (allowing the tool to fold in narrow for storage). The flange hole pattern is rectangular, not square, so turn the flange to position the holes farther apart in the direction where you want to have working space between the straps. Spread the straps apart as wide at possible while still covering the four holes in the flange (with at least 1/8 inch overlap to prevent breakout of drilled holes in the straps). Drill four 11/32" holes in the straps to match the hole locations in the flange. Select the wrench socket you will use on the big nut, and use that as a template to scribe arcs on the straps. Saw cut or grind away the metal back to the arcs (or a wee bit more) to provide wrench clearance. Deburr the parts to save your hands, and paint the bits if you like to be nice for future generations. (Optimistic, huh?).

If you do this job a lot you might consider welding a heavy steel ring on the working end of the tool with four permanently fixed steel pins matching the holes on the flange. Then you could slip the tool into place for quick use without needing to install the four bolts each time.

Addendum 6/18/2020:
If you make the steel strap a little wider for stiffness, maybe 1-1/2" wide, you can do it with only one strap.