Yes, you can shrink LUNs using SnapDrive and it is an online operation.
However do you know your underlying volume layout? Each LUN resides inside a volume, so if you have two LUNs, they can be in the same volume, or in different volumes. If the latter is true, then shrinking LUN1 won't release any space for LUN2, if you know what I mean - you may need to amend volume sizes accordingly as well.
SnapDriver also supports VMDK on datastore; and it definitely cannot resize datastore itself.
Note that VMFS cannot be reduced in size; so the only possibility to actually reduce space consumption is either use Storage vMotion to move data to new, smaller, datastore or enable thin provisioning and perform space reclamation.