It will be a bit tired... with that mileage the timing chain is stretched and causes some power loss...

 

whens the last time its had a good tune up (plugs, wires ect..)

and if the plenum hasnt been fixed, fix it. if nothing has happend with it yet, give it time. something will happen. get that done and maybe a quick tune up and go from there.

 

at 140k if it was gonna fail I think it would have by now and it would be burning oil if it had.... I have 150k and the plenum is one of the only things that hasnt failed... just verified when installing the new throttle body last week.. just saying...assuming either way gets us nowhere..
I'm gonna guess the OP is a new driver and just doesnt realize how sluggish these things are compared to newer trucks...I've driven a few 2nd gen rams and none were powerhouses compares to my dads f150 or my uncles chevy (both being much newer) the mileage may have something to do with it or the weight with 190-210 rwhp average pushing that weight with 3.55 gears.

 

you could have a blocked fuel filter its in the tank why does every one say its the pendlum if you dont ! have oil in your intake its not the problem how often do you think people drop there tank on a older truck 2 change the filter

 

I agree, my first thoughts were check the cat (which he said he did) and check the fuel pressure while driving. 142k isn't a huge amount of miles...helluva lotta Mikes, though. Mine has over 180k, and I wasn't greatly impressed with the power difference between it and a 2010 chevy with a 5.3 I test drove. Chevy had way more power up top. Bottom end, mehhh, not so much of a difference to me. If you floor these trucks in OD, they are dogs for sure until they kick down a gear or two. And they kind of seem to like to stay in OD anywhere over 40 mph unless your into the throttle pretty good. Is my observation anyway. So, to sum up, if it still feels gutless with OD off, might check fuel pressure to rule that out.