I believe 31" is right for an FM antenna. The cable to the antenna is coax cable. (ground shield around the wire) This is important too. When the antenna is bolted to the vehicle that ground is common with the body.
The hood on these squares is massive. The hood can loose it's ground through the hinges. FM reception is the only reason GM and others bolted a small ground lead from the hood to the firewall.
NOTE: The windshield antenna on a square is really tuned for AM radio, but it works okay.
Crusty is my 65 c20, and all three that I've had, didn't have ground straps from hood to firewall. Doesn't mean they didn't leave the factory without them.
I need to see if the 81 has one. I know my 91 cherokee does and 05 tahoe does too.
I never thought about fm wavelength and antenna height when I extended that antenna up, but it also didn't matter what height it was, no or very poor reception was all it got.
You pointing out ground, that antenna was mounted on the cowl extension, but that doesn't mean it had good ground. And that loose connection at the base of the antenna didn't help matters either.
Will check all of that on any future radio and antenna upgrades, regardless of vehicle make.
Normally would assume it will all work well, but need to verify nowadays and not assume anything.