A-518 shudder
 

Can anybody think of why I would get a slight shudder/vibration (mostly felt in steering wheel/gas pedal but feels similar to u joint going bad) ONLY under load in 3rd gear, going from 25-35 mph? To my knowledge this is not a lockup transmission (1990 dodge b350 maxi) and the vibration doesnt go away when you apply brake but it does go away when you downshift to 2nd gear. It mostly goes away when you let off the gas, but sometimes it will just reduce vibration. It will also go away usually if you just let of the gas repeatedly. Ive also driven it with the doghouse off and it doesnt look/feel like its caused by the motor shaking excessively (I cant really feel the shudder with my hand on the air cleaner while its happening).

Replaced ujoints, and nothing changed. I just bought the van but the trans fluid is at correct level, looks real clean, and the rear diff fluid is where it needs to be and looks good too.

Any idea would be much appreciated.

Check for bad U-Joints or center support...

That was my first thought, but I replaced the u joints and nothing changed. Its 2wd, and I replaced both and theyre spinning smooth now. It did seem to become less noticable when I replaced the spark plugs and fixed a vacuum leak so I think it may be engine related but I'm looking for any other ideas anybody would have...

In that case. I would say you started off in the wrong direction.... I think and complete service (Top to Bottom) and tune up would be in order as well as a carburetor overhaul if you have not done it in a long time or have no history.