Navigation says only proceed to highlighted

I wouldn't be too quick to assume what the issue is.

Just go into the diag screens on the MFD and select Navigation Check. From there, you can select GPS Information and Vehicle Sensors.

On the GPS Information screen, you can see the different satellites being received and their signal strength, and the quality of your location fix (3D, 2D, NG, etc.) This will likely tell you whether there's a GPS issue or not (which could be the antenna or the GPS receiver). Naturally the signal strength might be poor if you're in a place without unobstructed sky.

On the Vehicle Sensors screen, you can check whether the SPD value accurately matches the car's speed (be safe, maybe have a friend watch the screen while you watch the road), the REV value shows when you're in reverse, and the gyro sensor knows when you're driving straight (2.5 V, 0 or 360 degrees) or in a turn.

Then you might try going to the regular navigation setup screen and choose Calibration. There is a manual calibration for Current Position, where you adjust your current position and direction shown on the map, and also an automated option for after a tire change. You might try that too, even if you didn't recently change the tires. Everything's pretty old by now on a 2004.