Can I sell my mineral interests if they are currently under lease?

Yes. If your minerals are currently leased you can still sell the underlying asset. The leased property's value is partly determined by the royalty interest you currently are recieving. For example, if you have a 1/8th royalty interest the value would be 1/3rd less than a 3/16th royalty. Likewise a 1/4 interest would be valued at one third more.