The ignition tumbler is pretty easy to replace on 1995-up, providing you have a key that works as you need to turn it to get it out. You have to remove the column plastics; start by carefully prying the bezel off the ignition switch. Then there are some torx screws recessed into the lower one. It hinges away from the upper, and then there's some external torx screws going in underneath the top one; if you don't have any e-torx bits you can usually get away with a small socket on those as they aren't that tight. Now you've exposed the column casting itself and you'll see a small hole above where the tumbler sits inside. Disconnect battery, turn key slowly until you see a tab appear in the hole. Press it with a pick and pull the tumbler out.
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As mentioned earlier 1998 is first year for Passlock antitheft on these trucks; you need to get a tumbler for a 1998 as earlier years will not work. It has the resistor in the tumbler itself for the antitheft, as opposed to it being in the key like earlier VATS antitheft some may be familiar with from Corvettes, Camaros, etc. from years past (on those you can see the "chip" in the key.) You'll need to do a security relearn procedure since the resistor is likely different in the new tumbler. Try to start the truck, it should stall out immediately and flash the "security" light. Turn key off. Now this next part repeat three times: Turn it back to "on" but do not start it. Let it sit 10 minutes. That should do it...it's been years since I've done one so you may want to 'net search that one for accuracy.
Distributor gear is problematic on *some* of the Vortec engines as they were improperly hardened from the factory. I agree with the "ain't broke, don't fix it" mentality on these as you need a good scan tool to properly set the timing once you've pulled the distributor on one of these engines; you cannot do it with a timing light.
Lastly from what I saw in your original post. Limited slip additive is a no-no. If you've got RPO code G80 on the option label in your glovebox, this is a locker (Eaton Gov-loc) - it is not a limited slip or posi; those were not offered in these trucks. Per GM service bulletin you should not use any limited slip or posi additives as they can cause the Gov-loc to not work properly.
Richard