3rd gen DX trucks came with the tach, I am not sure if that was an option for the 2gn X-cabs.

If you remove both of the consoles and turn them over, you can trace the the electronics on teh back. IIRC, the oil guage from the stock harness has to be removed and put into another slot, which is easy to do. The Tach is also mapped out on the back of the SR5 cluster, you can run a wire from the tach tester under the hood to the cluster, and connect it with stock pins either from teh dealer or the wrecking yard, the yard will be cheaper.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed