Dodge Ram

I have a 1999 Dodge Ram 1500 with 360 and automatic. When in Overdrive/lockup at speeds above 45mph, if I accelerate slowly or climb a grade to a point where it would shift out of overdrive, the truck shudders.
I have not been able to determine if it is transmission or if it is engine misfire (I suspect transmission).
There are no codes in the system. Transmission fluid is changed every 30K miles, new plug wires (correctly routed and in correct order), new rotor, new distributor cap, and new plugs. I have 80K on the truck and the problem started about 3K ago and has gotten worse.
As soon as I accelerate hard enough to pull it out of lock-up or if I turn off overdrive, the problem disappears. Also, if I let off a little on the gas, it disappears.
I cannot duplicate it in another gear.

Ideas? In advance, thanks!
September 19, 2006.

Sounds like your overdrive gear is starting to go bad, does the o/d light come on when this happens? If not I would check all the upper and lower ball joints, u joints, transfer case

Sep 26, 2006.
Appreciate your thought. That has been my guess/fear. U-joints checked fine, no light, never thought to check the suspension points, but doubt them as it only occurs under load in overdrive while transmission is in locked up. As soon as I come out of lockup or back off on power, it disappears. 2 wheel drive so no transfer case.

Sep 26, 2006.