Max's answer is incorrect to your question “where do I start?”. Wheel balance is not the FIRST possibility to check which ( A side note; ASE test questions for professional mechanics often give several correct answer but look for “most likely” cause of the complaint.)
He is right about taking it to a shop although why people are always expecting us to work for free-SMH. Remember you get what you pay for and we remember what you spend and don't spend. We also talk with other shops about customers.
I am not being a jerk just straight forward.
Your question is very basic. It lacks important details but includes irrelevant details about tire size and axle replace. That indicates you have little knowledge of mechanics so, for you, where to start is by taking it to a shop for inspection ( which doing before purchase is a good idea and perhaps working the cost onto the seller). This problem could cause a serious accident. You want it fixed correctly and none of the possibilities are something you likely can fix. If you could you wouldn't be asking.
Second is to buy cheap automotive training books and studying them. The authors are verified experts as opposed to random people with questionable knowledge.
Find them here
Here is the best advice you'll get.
I am assuming you are young, so educate yourself and get a great income, save your money, avoid consumer debt and buy thing when you can pay cash for them. Don't do something you can't afford. If you can't afford to off-road because you can't pay for repairs than wait until you got the money.
Get your fundamentals right, have a career and income, work to increase your income, buy a house and have an emergency cash fund of 6 months expenses, have no consumer debt, anticipate future life events and when you have extra cash than you can do what ever you want. And if you want off-road I bet you'll be able to afford a nicer truck than a 30 year old ultra basic and small engine Samurai.
Then when life throws you curve balls you won't have to sell you toys.
Stop messing with stuff you can't pay for and wasting time, time you could be using to put money in your pocket, asking strangers questions the answer they may not know.
Here is the process for repairs: confirm complaint, visual inspection, diagnosis, repair, test.
And nobody online can possibly tell your the cause of the problem because that requires physical examination of the truck. Your description lacking detail doesn't help.
Is there any sign of vibration at speeds lower than 40 mph. Did you drive faster than 45 and the vibration stopped, slowed or became worse?
How sever is the vibration? “death wobble” and “nasty vibrating” is that to mean the vibration is severe enough that you risk losing control of the truck and shaking the steering wheel? Or just loud and annoying but you barely feel it in the wheel?
As to why Max is wrong about what is first to check. To check wheel balance require lifting the truck and removing the wheel. However before doing that you check visually for something obvious. Like a broken mount. (On my samurai the bracket on the axle which is where the shock attaches broke and that causes a similar vibration.)
Next lift the wheel and check suspension parts and steering parts for looseness. Checking the wheel bearing and tie rod ends which is done with the wheel on. Grab the tire top and bottom and see it you feel movement. And checking the ball joints. It wastes time otherwise and you don't want to miss something by assuming only one part is causing a problem.
Given the age of the truck it is likely you have multiple worn parts.
Get a set of street tires and a set for off road. Matching set.