went from 10 bolt to 12 bolt (3.08 to 3.71) What is the best way to access the speedometer gear to replace . Underneath our pull the console and retrieve it from the top?

In the trans, just disconnect the cable from the trans and the gear will come right out.

I've done them from underneath. Sometimes there is a small retainer screw you have to let loose first and that's the only way to really get at it.

What transmission?

Regardless of trans missission it needs to be done frrom underneath. the driven gear is easily accessed but the drive gear not so easy. Driven gear is retained by one bolt, the drive shaft and the tail housing need to come off to access drive gear.

Jeff

Would I likely need to replace the drive gear also?

Not necessary to pull the drive gear, but you must count the teeth on it in order to figure out the correct driven gear to go with the new rearend. To count the teeth you'll have to jack the rearend up and put on stands. Use a permanent marker to color one tooth of the drive gear. Then slowly turn a tire while counting the teeth as the gear also spins. Do that a couple times to get the correct count.
Here's the formula to figure out the driven gear size.

Driven gear teeth =
(# of drive gear teeth) x (Axle ratio) x (Tire revolutions per mile)
1000
Tire revolutions per mile =
63360 /(Tire outside diameter in inches) x 3.14159