2001 Jimmy-runs fine, was PO338 now PO300, and case relearn question

[JellyBeanDriver]

Not a truck but maybe someone knows what to do next?

Helping a friend with a 2001 Jimmy, 4.3L V6. Can't smog due to PO128 and PO338 error.

Fixed PO128 with a new thermostat.
Inspected CKP sensor and it is not hitting the reluctor but put a shim on it anyways. Now I have a pending PO300 code (random misfire).

What is interesting is when parked and I rev the engine up to ~3500RPM, the speedo starts to give a reading - like 4-6MPH.

Vehicle runs fine otherwise. Cap and rotor look good but also look original. Plug wires someone changed in the past. Look good but I don't know how many miles on them.

I'm thinking the PO338 to PO300 change is a clue. Want to do a CASE relearn but no scantool I own will do it (is there an inexpensive one?) Or is it red herring and the fact the speedo gives a reading shows the problem is electrical noise?

Anyone run into this before on this or another GM 4.3L product?