I wish you could take a picture of your skid.....if those small idler wheels are mounted to the front arm, something is really weird.
ALL of the Intruder model years used the same basic rear skid.
A couple differences...
The 78 skids had a different front spring design than all of the 79 and later skids.
The 78 and 79's came with the 3 wheel rear axle...you already know about that.
ALL of the Invaders were the same deal....EXCEPT the 81 Invader changed to the improved rear skid that first appeared on the 80 LTD.
With the exception of the 81 Invader, all of the other Intruder/Invader skids should interchange...bolt in and go.
Somehow, I fear you may have a really unusual rear skid that someone may have pieced together, if it doesn't closely match the pics of my rear skid.
Rick