Jeep Cherokee Forum - Low oil pressure. rough idle. jumping and clattering!!

So this is my first post. Ive never been a member of the forum (although I use it as my go-to for tech issues.) This is my second jeep cherokee, my 99 had to go into retirement because the frame rot became borderline dangerous. (which is for sale/parts, by the way)

I now have a 98 cherokee sport 4.0 6 cyl with a 5 speed transmission.

It drove great when I was bringing it home after purchasing it (about 100 mile drive.) and all it really needed were new wheel bearings.

I just put it on the road and gave it the platinum treatment. fresh oil change, new plugs, new diff fluids front and rear and a coolant flush.
On the first drive it showed low oil pressure and it was rough at idle.
I brought it back to the mechanic and he suggested a heavier oil.
With the new oil its still showing low oil pressure and jumping and clattering. I couldnt even drive it all the way home! Barely got it into 2nd gear and it wasn't accelerating and was jumping all over the place like it was about to stall.
I have no idea what the problem is and my mechanic seems pretty stumped (made me leave it there until tomorrow.)

Anyone else have an issue like this? Any help would be appreciated.
Thanks for having me.