why do I only get heat with acceleration in my 2007 Ford Explorer?

Coolant level is low, air in the cooling system, partially plugged heater core, worn out vanes on the water pump. Usually it is the first 2 problems.

coolant is full

Then there must be air in the system. I have had a few of these that would not clear out the air, I wound up putting them on a flushing machine to get it all out.

Sometimes raising the front of the vehicle will help during the air purge process.

If the problem only occurs while accelerating suspect a vacuum problem. There should be a pressurized canister associated with vacuum system. The purpose is to maintain vacuum when the engine accelerates when vacuum drops. Most have a backpressure valve which can stick.