I also loved driving my 47RH-equipped 1st gens with OD and LU on toggles - puts me in charge of them. So what you're looking for is something speed-related that will 'enable' the LU circuit over a certain speed, like 15 mph or so, and disable (unlock) it below, correct? I don't know if this would work, but if you have a 4x4 (NP205) I think you can thread one of the slick B&M Converter Controllers (i.e 70244 - link below) onto the t-case speedo output and wire it to a relay, which could 'enable' your manual LU toggle at whatever speed you'd want. The controller simply outs 12v signal at speed 'X' - B&M markets the thing to control LU on a GM but it could be used to turn on 'anything'. You can dial in whatever speed you want. Buddy of mine uses two of them in-line with the mechanical speedo in his 87 Ramcharger to control both the OD and LU - all automagic, and all adjustable. Not sure it'll work for your 93, but if so then coolieo.

There's a couple different versions of the thing - the 70244 is for mechanical speedos and the other for electric - maybe you can make one of them work? I have an Atlas t-case in my 02 big block Dakota with a mechanical speedo and the I'm considering using one to disable LU in my 46rh.

https://www.summitracing.com/parts/bmm-70244

- Sam