If it's not a piece that just snaps back in then it's going to have to be Super Glue to the rescue

 

Put the hips back where they would be in robot mode, does it still stay connected?

 

Not in vehical mode.

 

There should be enough pegs to keep it on...

 

Also, just incase no one knew this, the blue panels on the backs of his legs can pull out just slightly for a more movie accurate appearance.

 

Optimus.

Ironhide actually isn't that good. Sure, he's good for kids because of all the gimmicks, but for older people or collectors he's pretty mediocre. He has no articulation in a bunch of places where there could easily be some more articulation. Also his transformation is simple, but not in the good way. He ends up cheating and still having some kibble on his back and arms.

 

This.
Also for further proof, look at this:

 

This is why I hate it that Hasbro simplified the transformations. It gave us crappier robot modes in most cases and the robots have become shellformer-ish. Most of the DoTM figures have half of their car mode handing off their back. Roadbuster and Sideswipe are an example of this and the deluxe Soundwave too that will be released soon. The eradicons from the TF: Prime line also have car buttflaps too  
If Ironhide's transformation was more like Optimus Prime's, had darker grey instead of blue (Why do they keep this color when there's barely any blue on him?) AND he came with Heavy Iron 1.0&2.0 like he had in the game and movie, he'd be an instant buy for me.

IMO intricate transformations = Screen-accurate robots. There are several exceptions however that I can't think of off the top of my head at the moment though