While I wouldn't call this a 'jig' by any means, I did do my mini-tubs, the Qudra-link install, and the rear crossmember install while my Firebird was installed. We used 2" .125" wall square tube (a stick & a half or so). The wheels are the Harbor freight cheapies (use the bigger wheels next time). The axles are just 5/8" galv rod with a cotter pin hole drilled into the end. I used very basic dimensions (outside dimension of the rectangle): 36" x 72", which creates a 'bed frame' for the front & rear frames to sit on). I then took some scrap 1" and made some stubs that would allow me to bolt this sucker to the frame. We did have to cut a section out of the x-member that is under the engine / trans, but that was pinned back in.
Yeah- it doesn't steer, but I can pick the front frame up and manuever it like a wheelbarrow. The fact that it doesn't steer means that it doesn't move around on me while working on it (unless I bought some expensive locking casters). It has supported the weight of what you see plus my engine and trans, but the tires won't look like they like it!
So, $70 US worth of 2", $20 worth of wheels (4.99 ea), $10 5/8" galvanized rod (2' section), 8ea 5/8" fender washers (.12 each), and a case of beer for my buddy that finished the welding. Took us about an hour start to finish. For next time, I'd buy some bigger wheels, or put another wheel on each side of the stubs.