rattle in OD and when it drops to 4th on hills

I've had my 96 4liter H.O. AW4 NP231 Cherokee about 7months now I just changed tires to 30 BFG and have stock 3.55 gearing . I go up the same hill pretty often and after the tire swap i have to drop to 4th on long hills now . Well she starts at 60 ish at +/-2000 rpm's (guages) and in about a quarter mile the OD gets bogged down speed drops steadily to about 45 and a rattle (metallic) comes in faintly from I think near the tranny. I pump it and it drops in 4th or 3rd at aroun 3000 rpms and the rattl is abit worse so I let off the throttle and ride it out at 40+mph like a wuss:banghead:. What the heck is that rattle and how do I make it stop!?
BTW 124000 on the clock

It either ping or a cracked exhaust header, I think.

ping?? Sorry I'm a bit of a noob . I've heard that term before but if you could define that would rock. There is a bit of a knock at idle, but an old Jeeper said that's just the way a Jeep's idle sounds, like their trademark. Bull$*#t or not???

Anyone? ping definition? got new plugs wires cap and rotor>

Ping is a ticking noise when you get on it hard up a hill.

It is due to weak ass octane gas or a lean mixture or just too much tire for the gearing.

Put some mid grade gas in it and if it goes away...it is ping.

Thanks. I'll try going to higher octane this weekend up the hill and see if that does it. It is more of a metallic rattle kind of "blender on a tin can" sound. That's a ping? I'm just worried about the tranny. Fluid looks like crap, but am changing today>

Rattle could be an exhaust shield that's moving around with the lower rpm's. Check to see if anything underneath is loose.