1983 300SD TurboDiesel - Power Problems

[pantherpaw]

My 300SD has just over 300,000 miles on it...I've owned it for 1 year, and its been running great; however, recently, I cant get over 55 mph on uphill interstate driving...engine runs great around town and side streets but seems to be starved for fuel at highway speeds. The transmission shifts fine (but a liitle hard) through all the gears. I am guessing it needs both fuel filters changed, but i read the procedure in the manual, and was a little hesitant to change the filters because it says something about "manually pumping" to bleed the lines. Do my symptoms point to a fuel filter problem, and if so, is this an easy job?