Why Do I Have A Wheel Shake Even If The Lugs Are Tight

I have a 1971 dune buggy. Even though the lug nuts are tight I have a bad wheel shake and a clicking noise.

Dune Buggy Gas Cap

I have a meyers manx look gas cap is on hood under a cover.. Do they make a cap with a guage too?

What Is It Worth? I Would Like To Sell Our Re-furbished VW Variant

Cream color - very low mileage. Interior re-done.