99 Jeep Cherokee

When I am going 55 on the hwy and completely let
off the gas I feel a thud from what seems could be
the transmission but I feel the same kinda thud
when I first take off from a complete stop but never
if I don’t stop completely and if I ease off the gas
doing 55 I don’t feel anything just trying to pin point
what’s causing this noice I can hear and feel it’s not
real loud but loud enough to bother me ... the Jeep
does have 215k so probably could use a tranny
fluid and filter change but it shifts relatively smooth
through gears any help and or advice is much
appreciated