Neighbors 99 Ram 1500 with a 5.9
backfires through the throttle body.

I think he said while on the throttle and then leaving off

Thats terrible.
Is there a QUESTION you would like to ask or are you just sharing?

might look for a collapesed lifter,worn lobe,bent PR,loose/bad rocker,broke spring

I had a rocker back off and would pop back up thru the carb,was a bad exasht lobe on the cam(was a ferd)

may check to make sure the dist did not get turned or the chain jumped a tooth also

Like scatch said, worn lobe, bent pushrod, broken rocker.

hmm...what about an intake manifold plenum gasket leak causing an overly lean mixture coupled with sucking in oil?

I thought of that too,my sons has a leaking intake and it wont pop back up thru thru the throttle body.

does run like poop and blows a lot of smoke though

it would have to go real lean..no?

I've seen the minivans backfire through the throttle body. Usually on them a good throttle body cleaning does the trick.

I'd start with new mopar(champion)- plugs-wires-cap and rotor.

Does it only do it under load? if so take the upstream O2 sensor out of the pipe and see if it still does it. May have a plugged catalytic converter. Removing the O2 will let it run without backfiring