ram 1500 5.2 or 5.9 not sure vibrates very bad at certain speeds 1996

I have 96 ram 1500 with a V8
magnum engine, not sure if it's the
5.2 or 5.9 L. My check engine light
recently came on and ny mechanic
cleared it when he checked the car
out but recently I was driving on the
freeway and the check engine light
started flashing and my truck started
vibrating. I brought it to my mechanic
and he said it needs a tune up so I
got it a full tune up which cost me
400 bucks, drove it around and the
vibrating started again when i was
going steady 70-90 km/h, when I let
go of the gas it doesn't vibrate but
when I push it lightly it does. I push it
down a little more to get past 90 km
and go a 100 where the shaking
becomes a lot less worse but there's
a rumbling noise in my engine. My
mechanic looked at it for a few days
and told me one of the cylinders is
not pushing what it's supposed and
he needs to change the piston ring?

Anyone have any ideas here I dont
wanna get screwed over.