For backspace/offset questions, I like this site. Utilizing this site, I don't need to "be able do 3rd grade math" to convert off-set (f'in PITA way to measure rims) to back space (much more consistant). 1999-2004 can use 4.25~4.5 back spacing, IMO, anything deeper is going to create more contact issues.
Custom rims, wheel tire packages for your ride - RIMSnTIRES.com
I used 365/75-16 to simulate the 38x14.50s you were asking about.
Depending on the backspace/off set of your existing 18x9s, you could use that as your base line. I have run 14.5s on a 10 inch wheel with about 4.5" of back space, they rubbed at near lock the leaf springs near lock. But I only have a 4" lift and those were 35s, so your results may vary.
However, having seen those same 14.5 Toyos on 12" wheels in my friend's garage, I wouldn't run 14.5s on anything less than a 12" wheel if I had the chance to do it again. Like a dumbass, I rolled those Toyos at 50psi all the time, well, the centers were nearly bald and the shoulders still had plenty of tread.
We have 38X15.50-16s on our 1/2 truck, to get a decent tread patch on a 10" Weld, I have to run them at 25 psi.