1993 Ford Ranger valve chatter under load

Hi, my Ranger makes a pretty proufound valve chatter/clatter under load, mainly going up steep hills at highway speeds (45 to 50), and losses power noticably with top speed of about 45MPH. If I down gear it subsides only slightly. Under normal load it drives fine and only makes occassional, light clatter noise when excellerating.

My questions are- how critical could this problem be? Is it indicitive of needing an entire valve job or top end. Or could a simple valve adjustment remedy the problem? For that matter, can the valves on a 93 4.0 even be adjusted? Am I likely to cause major engine damage if I continue to drive it without addressing the problem? Like I said, I hardly notice it under normal driving conditions.

Many thanks!