I have a 99 4.0 sohc 2WD Explorer. And under low vacume conditions (going uphill on the hi-way even w/ cruise control on the dash vents switch to defrost) I know its a vacume loss problem just can't locate.anyone come across this same problem.

Check the vacuum canister (little ball shaped thing on the passenger inner fender), its possible the hose came off or the canister is damaged and not holding enough vacuum.

I don't think the '95 and ups have the reservoire canister. I can't remember ever seeing one on my '97 (not that I've looked for it though) but when I looked over the A/C diagrams though for my Explorer I don't think Ford uses a reservoire anymore. If that turns out to be the case, then the likely problem will be behind the dash where most of the vacuum connections are made. One has probably popped off or a hose might have cracked. Look on the fender first though since that would be far be the easiest place to start.

Also do a search for "A/C", "vacuum", "defrost" and anything else that might be a match. There were at least two other threads that went into great detail about this problem. I believe both people found the source of their vacuum leaks.