Thank you for the opportunity to be of assistance.
The 10.00-20 tire is designed for use on a 7.5" rim.
It is, however, allowable to use the tire on a 7.0", 7.5", 7.5M, or 8.0" rim.
The 11.00R20 tire is designed for use on an 8.0" rim.
It is, however, permissible to use the tire on a 7.5", 8.0", 8.50VM, or 8.5" rim.
Therefore, if your vehicle currently operating 10.00-20 tires is equipped with either 7.5" or 8.0" rims, it is possible that it would be acceptable to run 11.00R20.
However, you stated these are rear wheels, and we are assuming they are dual assemblies, in which case there is a second consideration.
The 10.00-20 tire requires a "minimum dual spacing" of 12.5".
The 11.00R20 tire requires a "minimum dual spacing" of 13.0".
Therefore, while it is possible that this may work, we would highly recommend that you have the installing tire dealer confirm that the above requirements are met.
You should also consider that the 11.00R20 tire will have a substantial difference in overall diameter (about 1.3") vs. the 10.00-20, which will affect vehicle clearance, actual final gear ratio, and speedometer/odometer accuracy.
Best regards, Tire Doctor