I've had this 98 XJ for several years, fixing it's issues a little at a time.

One thing it's been doing since I bought it, is this weird single knock that comes from under the driver's side of the dashboard.

It usually happens just after I put it in drive to go home after work, and before I move more than 50 feet.

It's just a sound like someone hitting plastic sharply (just once) with a screwdriver or wrench.

The only thing I can think of is the plastic cover over the top of the gauges is hot from being in the sunlight and then cooling suddenly because the window is down. Maybe the whole thing contracts a little bit and moves at it's attaching screws.

I have the same issue.

I leave a few pieces of Kittles and Bits on the driver floor each night and they are gone in the mornng, so I'm wondering if......

Could be an HVAC door closing.

Did it again today, weird it only does it once when the dash is in the sun and got hot.

Mine does the exact same thing. Usually a loud Crack when I start driving. It happens once per trip for me as soon as I start moving, very odd. I kind of wrote it off as the dash settling?

Same here. I've been tossing the idea around of getting some felt tape and running along the inside edges of the dash panels to see if it would fix it.