97 Gallant ES Tranny Problem

Have any one had this problem before? All the other gears work fine but if I put in "D" I will get two shifts up to 40ish MPH and the last shift will not go...the RPM will just shoot off the chart. Also, now the check engine is on. I have tried adjusting the "adjuster" (on top of the automatic tranny below the air filter) but I am just guessing since I have no idea of properly doing an adjustment. Should I line up the two small holes in "N", "D", or "P" and make adjustments from there? Or is it possible that my tranny is toasted? Again its a 97 Gallant ES (Auto) with about 66,000 miles. I have another 89 Gallant beater and its still running strong with about 150,000 miles so I hate to see this one with only 60K+ crap out before the old one does. Any help will be greatly appreciated...