Is there a speedometer cable? If so, find where it goes. If not, it is electronic. If it is electronic, no gear. In which case it is probably using a sensor in the rear diff pumpkin.
My '88 had a speedo cable and a gear. It was in the rear shaft housing. Had a spiral plastic gear on the shaft, and a spur gear on the cable. You couldn't miss the big speedo cable. So if you are not seeing it, I'm betting a '90 does not have one.
Folks call me Bill. Why, yes, it is a 6 liter. Your jealousy is showing.
I found another post on this site about it, here.
"For that vintage truck the speedo and RABS systems are seperate, the speedo is cable driven off the T-case and the rear axle sensor feeds the RABS system only. Somewhere around 1992 Ford integrated the two systems and did away with the cable driven speedos, you can tell what system a truck has by looking at the odometer, if it's mechanical then the dash is cable driven but if it's LCD it is electronically driven from the combo RABS/VSS sensor in the rear axle."
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