I have been getting vibration at low speed around 30 to 40 mph mostly in 3rd gear

. Check front and rear universal joints on drive shaft.

. Check level and condition of transmission fluid. Drop and clean transmission pan and magnet, change filter, and refill with Mercon V ATF, if it has not been done in 30,000 miles.

. Check the level of the gear oil in the differential. Change every 100,000 miles.

. Check for tire wear/balancing.