Botched rear end replacement... Even more questions!

Ok, so the guy at the pull yard was adamant the rear end I got was for my truck...  The rear wheel drive is fine.  Things I noticed:  It''''s 5MPH slower than what the speedo says.  And when in 4WD it''''s in a bind.  Conclusion: Wrong damn gear ratio.  So, is it possible to correct the situation, that I can just swap the front axles from the truck in which I pulled the rear end???  Final question...  How many ratios fall under the part number GU6?

One.  3.42 = GU6. 

If the guy gave you a GU6 rear end, it was probably swapped at some point in its life.  Or it wasnt ever a GU6 rear

You could swap the front...but thats a little bit more involved.  Also, no way to know if the truck youre pulling from has matching gears!  Could be the guy blew his rear end, replaced the gear for cheap, and never used his 4wd.  

Well, that sounds very possible since there is only one!  Damn, I didnt know this.  So I do have to replace it again then.  That sucks.  How can I positively identify one at the PY then?

Pull the diff cover off and count the teeth on the pinion and ring gear.

So the PY owner tells me I may be able to match the rear diff with another choice of front axles...  Will this work? 

Yes it will, but I would rather get the correct rear axle than have to dig into the front diff.

Yes, it will.  It will leave your speedometer incorrect though.  And I would definitely rather get the proper rear end than mess around with the front axle.  It is not the easiest thing to swap out.

Yeah, it''s 6.5 MPH slower than what the speedo says...  Does that mean higher gear ratio?  Gas saving?  lol  So is are the front axles really that hard to swap out?  He says it''s a sinch.  The rear was super easy, I just don''t feel like doing all the brake stuff again.

UPDATE:  I''ve decided to get another rear. 

Good idea