My truck skips(thumps) when at speed of 60-70MPH, what coult that be from? on 2006 Lincoln Navigator

When driving at speeds of 60-70mph my truck skips (thumps) and I am not sure if the truck has a timing belt or not or what is causing that... I just had a oil change, air filter replaced and transmission flush...

Asked by for the 2006 Lincoln Navigator
Check the rear strut or shock to see if any leak
Thanks Nguyenk... I will have this checked. Thanks for the follow up.