"I don't know why in the hell the shop folks would tell you that valve guide issues would have been caused by the rear end." .... because the engine is being over-revved running 55-60
and swapping the 4.56 for a 4.10 won't solve that problem, only cuts your RPM by 10% at most, lots of work and expense for no gain - as David says, you need to get much better gears than that, and you'll probably only get it with an OD, not sure you'd find 3.55 gears for that rear
Bill
