2002 f150 lariat fx4 228,000 miles. 4x4 light comes on, u can feel the rear end change gear ratios, but the front wheels dont spin or have no power. I dont know much about four wheel drives and dont know where to start. Is there a chance it's just low on transmission fluid? Any ideas?

 

check to make sure the system is getting vacuum at the solenoids and at the CAD on the front diff.

 

Just an FYI, the rear end is not changing ratios. What you are feeling is the transfer case engaging the front driveshaft. From there, the front diff is then actuated by vacuum, as djfllmn stated. The solenoids sit right up against the firewall, behind the side of the battery. It is pretty common for the battery to leak a bit and have it rot/eat the vacuum line up there. That is where I would start, then work your way down as djfllmn suggested.... all the way to the actuator on the front diff.

 

Here's a pic of my spare front diff - it does NOT have the actuator on it, but it goes where I've circled in blue. The swing/engagement arm is circled in red. The actuator engages that arm via vacuum, supplied from the solenoids up by the battery..... It is not a very long swing, but you will be able to see it move if it is working properly. If this DOES move, than you've got an internal issue in the front diff.... but that is less common than a vacuum or solenoid issue.