You can use a whip with a coupler, but it must be one designed for that use, like an Icom AH-4. The LDG you mention is NOT so designed. And the coupler must be within INCHES of the feed point of the whip - no coax at all on the connection.
Additionally, a 108" whip cannot be effectively matched for 80m operation, so if this is your intention, it's a bad idea.
If you arer SERIOUS about mobile HF, a _full_size_ morotized antenna is the preferred way to go.
I suggest you take a look at www.k0bg.com
for more information.
Lon - W3LK