There is no distinction for ground or air use of 2.4 under FCC regulation in the united states. There is as noted a distinction in the VHF bands 72 for air use only and 75 for ground usage only. Also there are six channels in the 27 band that are combined usage.

There are a number of 2.4 module/Rx systems being used for conversion of certain older 72 radios and this is perfectly legal and the systems from the majors are certainly not hack modules.

John

I neglected the two bands on the ham bands requiring an FCC lisense are also combined usage ground and air.