I added level shifters to the I2C SDA and SCL on the MPU9250 and I added them to the TX and RX on the GPS and the Serial TTL radio.
Adding the level shifters has substantially improved the MPU issue related to SD writes, but not eliminated it altogether. I used to get a crazy reading about 1 in 10 reads followed by a SD write with no delay following the write. Now I get a crazy reading about 1 in 100 reads followed by a write. Also, before I had to use a 27ms delay following a write to the SD Card to fully eliminate the issue. Now I can fully eliminate the issue with a 10ms delay. So, the level shifters definitely helped a lot, but odd that I still get the issue. 10ms is manageable for my project and I've got a load of software filters to detect anomalies, but it still makes me question the integrity of the sensor.
I was also getting some odd lock-ups with the CPU before the shifters when the radio antenna was near the Arduino. I am using strong radios (2W), so I assumed I was getting too much RF interference on pins or directly to the CPU on transmit. The shifters appear to have fixed or substantially fixed this issue, as well. thanks!