It will not hurt to check your axle ratio.
The fluid in most can always stand to be changed
![Wink](http://67-72chevytrucks.com/vboard/images/smilies/wink.gif)
The Transmission is Most likely a SM465.
Unless it has been swapped over the years.
Link Below for SM465
The gear ratios for that transmission
are as listed below (According to Wikipe.)
![lol](http://67-72chevytrucks.com/vboard/images/smilies/lol.gif)
1st is 6.56
2nd is 3.58
3rd is 1.70
4th is 1:1
1st gear is great for rock crawling or pulling down a house
There is so much difference between the Ratio of 1st and second gear and axle swap will not make much noticeable change.
Even if (BIG IF) you had a 4:11 rear axle and swapped to a 3:08
Here would be the results per on-line Ratio calculator.
With 4:11 @ 2000 rpm with 31" tires
1st gear= 6.84 mph
2nd gear= 12.54 mph
3rd gear= 26.41 mph
4th gear= 40.81
With 3:08 @ 2000 rpm with 31" tires
1st gear= 9.13
2nd gear= 16.73
3rd gear= 35.24
4th gear= 54.46
With 4:11 @ 2000 rpm with 33" tires
1st gear= 7.29 mph
2nd gear= 13.35 mph
3rd gear= 28.11 mph
4th gear= 43.45 mph
With 3:08 @ 2000 rpm with 33" tires
1st gear= 9.72 mph
2nd gear= 17.81 mph
3rd gear= 37.52 mph
4th gear= 57.98 mph
Link to info on SM465:
https://www.novak-adapt.com/knowledg.../manual/sm465/