My 1995 Silverado K1500 (5.7 liter, V8) with only 22,300 original miles suddenly started to refuse to shift into overdrive. I noticed that doing 60 MPH on the high way it will run at a high 4,200 rpm. If I shift manually into 3rd gear and back to OD there is no change in rpm. Checked transmission fluid level and is OK. Any ideas what could be wrong or what to check for? Thanks in advance for your opinions!