A dsl thread is almost as bad as an oil thread. Lotsa opinions. Lotsa variables likeinitial cost, ease of install, mods needed, scarcity.
I'm a big fan of either Cumins motors (I prefer inline dsl's) and the 6.2 GM. The later is prolly the cheaper route but Isuzu does build good dsl motors too and they designed the original GM Duramax v-motor. I also like most of the MB dsl's but I think the only ones in the US that have enough power for a FSJ would be one from a Sprinter van but think $$$$$$$$.
What's your priority? Cost, easy install? Anyway way you go is expensive and none of them are drop-ins.
I would't base my motor choice on a trans I have laying around and want to use. Buy a complete donor truck.
"Don't mind me. I'm just here for the alibi"