It will not hurt to check your axle ratio.
The fluid in most can always stand to be changed
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.)
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/