More often than not, when we start up our truck and pull out, the engine roars and feels like it's not shifting. After about 1/2 mile, it seems to shift into gear. It has been doing this since we pulled a 16-ft fully-loaded moving trailer cross-country. It also seems to have wreaked havoc on the gas mileage, though that may be an unrelated issue. Aside from that cross-country trip, it doesn't get driven much. One mechanic mentioned something about changing out a fan? Any suggestions?