Crankshaft

Just had my transmission rebuilt two weeks ago and drove the (97 voyager 3.0 engine) van on a trip of only 400 miles last weekend.

I noticed a "thumping" sound which was accompanied by a slight vibration when driving at speeds between 40-60 mph.

It almost sounded and felt like I had a tire whose radial belts had seperated. I checked out the tires (they were fine) and drove the van back home.

When I checked it out at home I noticed that the pulley which goes to the crankshaft was the "thumping" I heard. It looks like it is not wobbling to the timimng of the "thump."

The pulley seems to be tight. Is it possible that the crankshaft is bent? If so, would this have anything to do with the transmission I had rebuilt?

Correction. The pulley IS wobbling to the timing of the thump.

It is doubtful that this has anything to do with the transmission since they are on opposite ends of the engine but it could. First, check to see if either the pulley has broken or the harmonic balancer is going bad. If you are not mechanically inclined take it to a repair shop to have it checked out. If you have the tools, jack the van up either with ramps or put it on jack stands.
Remove the serpentine belt and crawl under the van,
check the balancer and pulley with your hand, if the bolt is loose the entire thing should wobble when you shake it. You may find some play in it, if you do try to tighten it with a socket and rachet. It is the large bolt in the center. Also check the bolts on the pulley, they may have vibrated loose and be causing the entire problem. If so simply tighten them, replace the belt and the problem should go away. If it continues to wobble take it to a shop right away and avoid a lot of driving.
The transmission could also be at fault since it was replaced. If the shop that rebuilt it could have forgotten to tighten one of the bolts holding the torque convertor to the flywheel or they may have forgotten to tighten the bolts on the flywheel and this would cause a significant vibration to occur. If you continue to drive it you could cause serious damage to both the engine and transmission. To repair this, they will have to remove the dust cover to check it and if the flywheel is loose they will need to remove the transmission again. Good Luck!

Thanks for the great tips!
I am going to have the shop that re-built it look at it Monday.

By the way, I crawled up under the van, didn't take the belt off, but checked the pulley bolts -- they seemed tight enough to me.

Then definitely have the tranny shop go over it again. If there was no problem with the engine before the tranny rebuild then that is where the problem is coming from. Once you identify the problem, let me know. Thanks!!!