Do you have a leaf spring axle or a Torflex type axle?
To replace a leaf spring axle you will need the overall length between the hub faces & the center to center distance between the spring perches. You may have to have the spring perches welded on if your spring spacing is not a standard width. I replaced both axles on my TT and had to do it that way.
I am curious though, if you have a 3500lb trailer, how have you been getting by with 13" tires? Also, what is the number of lugs on you existing hubs, 4 or 5? If 5, it should be 5 on a 4.5" bolt circle. You can simply replace the wheels & tires with 14", or even 15" if there is room inside the wheel well. If you have 4 lugs, or a smaller circle, then replace the hubs/drums with 5 bolt 4.5" bolt circle hubs and then the wheels & tires. No need to replace the entire axle/brakes in either case.
Jim & JoAnne
2016 Toyota Tundra SR5 TRD Double Cab
2004 Starcraft 11RT (Replaced by)
2011 Eclipse Milan 18CK Travel Trailer