Installing gears..

I think you ok. Some may say a chip if it is auto because the
tranny is gonna want to shift and the different gearing is supposed
to effect that some how. If you standard you good for sure. I hope another can assure you if you have an auto.

The mechanic can help you make the call on other parts that might be needed. Go to someone that has experience with the 8.8" rearend - they'll know what to do. I'd replace the pinion seal, it'll take a new crush sleeve, and they may/may not want to or need to replace certain bearings depending on age, condition and whether or not they mess them up when they're pulling them off. They will also need certain shims to set the clearances up correctly. So it's really important that you go to someone that knows and has set up those rear ends before. And you're correct - you'd almost be crazy not to have them freshen up the traction lock while you're in there.

you're going to have to find a shop that is able to install gears... they're relatively simple but they do require that you have 2 specialized gauges... one for setting the correct pinion depth and one for getting the correct drag... while you're at it you should get a gear installation kit complete with axle seals, new wheel bearings, and carrier bearings. Since you're only going to do this once in your lifetime, you might as well freshen up the whole rear end... its only about $100 for a kit. Also you'll need some 70w gear oil and friction modifier. If you bring all the goodies to the shop they wont charge you an arm and a leg.

As far as the speedo gear, you simply unbolt the speedo cable from the rear of the transmission and pop the old gear off and stick the new one on and bolt it back in with a little high temp RTV around the metal surface being careful not to get it in the tranny... also there is an o-ring that you could replace at that time so theres no leaks... autozone and kragen sell o-rings... just take the old one in...