i am going to be running 35" tires on my 98 f150 and wanted to know if i should re-gear it or not.

Go ahead and put the tires on and see how it feels. See if its sluggish off the start, punch it a few times to see how much power you can put behind the wheels. Now you might end up still having to re-gear it. My buddy has a 79 blazer on 38's and he had to re-gear it but my other buddy have a 01 chevy 2500 and he has 35's and never re-geared and can still lay a wheel on demand.

maybe 3.73s ?

i am going to be running 35" tires on my 98 f150 and wanted to know if i should re-gear it or not.

35s should be fine, i doubt you will ever have a prob with the gearing, but if you would like to spend the money 410s are a good ratio for off roading.

i have a 78 250 with 4.10 gears 35's and a 302w/c-6 trans
it drives ok would be much better on take off if i had an odrive.

I have 35's on my truck and I re-geared it to 4.56's. I got lots of power back as well as gas mileage.