Well, trailer axles come in all kinds of bolt patterns, so it would be wise to make sure it fits before setting out counting on that it will.
2nd thing to address, you need to make sure the back spacing is correct. Not for bearing reasons, but for clearance reasons. Trailers generally have their tires much more closely spaced to the frame, a 1/2" too much backspacing and you'll be up against the frame and not even close to the back of the axle flange. My trailer has less than 1" between the tire sidewall and the trailer frame.
3rd-ly, make sure the center hole is the correct diameter. Again, axles came with different types of centering hubs.
My jeep rim spare for my truck (the 14" original spare's center hole was too small for the 31 spline 8.8) fits on my Ranger perfectly, but it's too deep for the trailer and hits the frame and still has about an inch to go until it hits the flange.
For what it's worth, I run a set of 235/75R15 XL load rated LT tires on my 18" dual axle trailer, the exact same size on my Ranger. I got the tires from the junkyard for $20 a piece and mounted them with my manual tire changer (about $50-$75, and pay for themselves quick!), balanced them with higher end 20 gram airsoft BB's.
That's pretty cheap for a tire. Some people have issues with running non-trailer tires on a trailer, but I have noticed no ill effects. If anything, you'll have better grip in wet/snowy conditions and better stopping distance. It might be a good idea to grab a spare from a junkyard with measurements in hand of what fits your trailer (just be sure to tell them it's for your RANGER). And if it is indeed the same pattern as your ranger, them you'll have 2 spares that you can use and interchange.
Better to have it and not need it rather than need it and be screwed.
I have a little doughnut spare that I keep with my tow dolly.