CASEY-IN-OKC

I JUST BOUGHT THE TRUCK 3 WKS AGO W/ 83K MILES. IT IS 4X4 TEXAS EDITION X-31 OFFROAD. IT SEEMS TO BE SLIPPING BETWEEN 2ND & 3RD GEAR, AND ALSO BETWEEN 3 & 4. IT IS VERY SUBTLE. JUST A QUICK SLIGHT TICK-TICK-TICK BETWEEN 2 AND 3, THEN JUST ONE QUICK TICK BETWEEN 3 AND 4. CRUISES FINE. NO WARNING LIGHTS ON THE DASH. IDK IF IT WAS DOING IT THE WHOLE TIME SINCE I BOUGHT IT, AND I JUST DIDN'T NOTICE IT? OR MAYBE IT JUST STARTED LAST WEEK. IT IS SO SUBTLE, ALMOST FEELS LIKE IM RUNNING OVER CRACKS IN THE PAVEMENT. ANYONE ELSE EXPERIENCE THIS? ANY IDEAS WHAT IT COULD BE?