Hi all, new to the forum. 1998 XL 4x2 3.0L V6. 462K miles on it, but the transmission was replaced at 423K, still under warranty. Live in Colorado and usually driving in 5000-7500 ft elevation if that matters any. When driving, at around 45-55 MPH it starts to shudder and hesitate. I hit the gas more and it shifts to the next gear and goes away at about 55 MPH. It doesn't happen on deceleration. Plugs and wires are fresh, no rotor and cap to replace. Short of a rear oil gasket leak, everything else on it seems to be fine. Anyone have any idea WTHeck is going on and what to do about it? Is this something with the transmission or throttle clutch/converter (I'm not even sure what the correct term is!), or something else? All help and advice are greatly appreciated.