Its been 18K since the tranny service. Did it whine & shift hard right after the service or is this a recent thing? Burnt or low fluid can produce hard shifts. Pull out the AT dipstick after driving for a while and examine the color, smell, and level (follow owner's manual instructions for checking fluid - best is with engine running, in Park and after going thru all the gears).
Do not get a trans "flush". If you want to replace the fluid, just do a drain & refill & filter, not a flush. If you do it yourself, you'll only be draining the torque converter and replacing about 25-30% of the fluid. If the fluid is bad, do this every few months, eventually bringing the fluid to mostly new, until the fluid is nice & pink.