jeep lover

Our '98 5.2L Durango 4x4 has been a really super car, with very little maintenance issue. It currently has 138K miles and everything except the power steering pump and fan clutch is original. Lately, the AC fan works on occasion, but normally does not. It it is put on vent or AC with fresh air, about 30 mph the fan starts blowing. Sometimes when you hit a bump the fans starts working. Next thing you know, it has stopped, but at speed the AC on fresh air still pushes out cold air. The rear unit works fine, puts out cold AC. Also, the lower speeds on the fan do not push air out, only the upper two speeds. I suspect fan motor, does that sound right? How much of a job is that for a DYI guy?