Hello Allen,
Thank you for your post. Since your environment does not appear to support unconditional magnetometer use, that places more emphasis on the gyroscope and accelerometer performance. I would probably start with consideration of the ADIS16485, with a plan to develop application-specific angle estimation routines. Another approach could be with the ADIS16480, which provides angle estimates. This approach would likely require characterization in your specific conditions. It's EKF was designed for zero initial conditions/slow-moving inertial motion & the majority of its characterization and application use (to date) assumes magnetometer availability. I hope that helps!
Best regards, NevadaMark