would like some input on which size gear to get. 06 blown scab with ar 22", came with 355 ls, dont want to run out of motor at 90mph either. also how hard is it to switch them out myself?? particular gear oil i need?

 

With overdrive, you won't run out of motor at 90 mph no matter what you put in there. What you put in depends on what size tires you are running and how much bottom end you want, balanced against expected fuel economy. Follow the gear manufacturer's recommendations on oil.

 

you can do it yourself but if you have never done it before and have no clue, get someone to help you that does, and youll need some special tools and a word of advice, get a tuner and an intake and leave those gears alone, 3.55's are'nt the best and strongest but for what your doin they will be fine, if you decide your still a lil slow off the line, maybe upgrade to 3.73's, past that...... if you still want more power and speed an f150 is not the vehicle for that

 

Drop some 4.10s in there and be done with it.
I did that to my Expy, running 285/60R16 tires, and it was perfect.

Lots of guys on street tires turn 4.10s, including Lightnings.