actually they didnt. Check your door jam sticker. If your axle code is A03A then you have 4:10. Actually the most common 3rds are the 4:10. Toyota put 4:30 in only a few 4runners.
Its just the 3rd not the axles housing
Axle codes are nearly worthless in obtaining gear ratios on any Toyotas later than the early to mid 90s.
POWERTRAIN
Drive System Type
2WD front engine rear wheel drive and 4WD
front engine with shift on the move 4WDemand
2.7-liter 2WD and 4WD with 5-speed manual
Ratios 1st.........3.954
2nd.........2.141
3rd.........1.384
4th.........1.000
5th.........0.810
Reverse.....4.091
Differential Ratio.....*4.100 (2WD 3.727")
2.7-liter 2WD and 4WD with 4ECT
Ratios 1st.........2.804
2nd.........1.531
3rd.........1.000
4th.........0.705
Reverse.....2.393
Differential Ratio......*4.100 (2WD 3.909+)
3.4-liter 2WD and 4WD with 4ECT
Ratios 1st.........2.804
2nd.........1.531
3rd.........1.000
4th.........0.705
Reverse.....2.393
Differential Ratio..."+3.909 (2WD 3.727")
3.4-liter 4WD with 5-speed manual
Ratios 1st.........3.830
2nd.........2.062
3rd.........1.436
4th.........1.000
5th.........0.838
Reverse.....4.220
Differential Ratio ...."3.909
* = 4.556 w/ optional P265/70R 16 tire package
" = 4.100 w/ optional P265/70R 16 tire package
(standard on Limited)
+ = 4.300 w/ optional P265/70R 16 tire package
and all models with locking rear differential
Taken from:
http://www.top4runners.com/runhist/runhist319972.html