My kitchen and bath counters are separating from the wall. Do I just need to replace the caulk or do I need to replace cabinets and counters?

Without a picture it's hard to say.
Most often one of two things is happening the floor is starting to sag, simple to check just lay a level on the floor and counter top.

Depending on how the top or back splach are attached to the wall it could be the screws were to short or missed the studs or if it's not all one piece the glues reliecing from the wall.

How wide is the gap between the countertop backsplash and the wall behind it?