Mike - as for highway cruisin, all trannys have a 1:1 high gear, so the 4 speed is the same as the 3spd for highway mileage - w/ that 2.29 rear, the 4spd 1st wouldn't be a granny, it'd be a real 1st to get ya movin!! shoulda done one swap 1st to see the effect, then the next swap if necessary
best bet now is to change rear gears as that'll be easier and cheaper than swapin trannys till you find that isn't the problem!

as said, the 2.29 is waaay wrong for a truck, need more like 3
Bill