You right, the speedo runs off an electric signal from the back of the transfer case. There's a small gear turns off of the TC output shaft. I would start by pulling that out (pretty easy) and taking a look at the gears to make sure they aren't worn, then try tracing the wires for shorts or bad connections. Also check the back of the guage cluster for bad grounds. There are four screws holding the back trim on and some clips. It's a little tricky to manuever out.
You should be able to find a speedo pretty cheap in a junk yard, or even consider doing a guage cluster swap if you don't have the tach/oil pressure/temp/voltmeter and if your son plans on wheeling it. The extra info about the engine is nice to know.
I got a cluster off ebay for $30 shipped and spent about $35 on sensors to convert it over.
'00 TJ Sahara (RC 4" LA kit)
'06 Grand Cherokee Limited
'07 Ram 1500 quad cab
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