If you have an auto, I wouldn't go higher than 3.73s. If you have a 6M I say go with the 4.09s. In an auto with 3.73s, 75mph will be about 2600rpm. With 3.54s, 75mph will be about 2450rpm. In a 6M with 3.73s, 75mph will be about 1800rpm. With 4.09s, 75mph will be about 2000rpm.
And if your wondering, most stock tachs read a little high (300-500rpm).
You might want to read my question in the past history about Dana44-4:09.
I installed this and by the way love the 4:09 axle and anything less with a 6 speed is going to be a drag just using the 6th gear unless you are at say 100mph. Who drives that fast?
You still need to consider the speedo gear.