PM a fellow that goes by 'snarl' on the HAMB - he's fluent in the electronic speedo game (I'm not - I avoid them). Certain ones can be adapted to mechanical usage, and others cannot.
And you can find electronic speedos as early as '89, but 89-90/1, you can find both mech and electronic - by '92 they were all electronic (all the S10 ones anyway). I believe, but cannot prove (anecdotal evidence only) that the electronic ones in '89-91 were supplied when you got the digital dash upgrade on the top-of-the-line model...