I'm pathetic at diagnosing mechanical problems but...when my 1992 4.0L gets to temp and when under load it sounds like the valve train is flying apart and she caughs huge clouds of oily smoke, back off on the gas and ease it back in...no noise and full power...runs like a dream 95% of the time but when I lug the engine...she complains. I'm pretty sure it's the exhaust valve guide on #6 (plug was fouled when I bought it).

Can I get an expert opinion? If it is the valve guide, can it be replaced with the head intact, or just pull it and machine everything?

its possible to replace the valve seals with the head on, but not the valve guides.

This is a tough one but it does not sound like a valve train problem........it sounds like inefficient combustion and preignition (ping). Something may be wrong with the ignition system (timing it too advanced), or the knock sensor not working.