What are you swapping the cluster into? Something older than 95 I'm guessing, if it has a mechanical speedo. In theory the 95's just used the VSS signal which would be the same on a Gen 2/Early Gen 3 truck, so you could try tapping into that. However, 98 the signal changed and I don't know that it'd work.
I swapped in an Explorer Message Center into my 93. Stock it used the electronic speedo signal. I first tried the 98+ style center, didn't work (until you hit 30 or so, then the signal got to levels replicating the signal it was expecting I guess), but the 97 I put in works perfectly, so I would wager that you could make it work.