[KOG]

It won't be in the transmission. A u joint beginning to seize in the driveshaft could cause this problem.

 

[ImperialCrown]

+1 on U-joints. Put a pan under the tailshaft housing as you may lose some ATF, mark and remove the driveshaft.
Articulate the U-joints up/down and left/right, it sounds like the needle bearings might be binding.

 

[Rickorino]

Many times when u-joints go bad you can see powdered rust on the u-joint itself.

 

[rcnesneg]

I don't know much about U-joints, it may be them, but the most obvious thing, is tire balance. They may have thrown a weight or something. You said you checked that though.

 

[68RT]

With the mileage you have, I would think you have a u-joint going bad. No change in vibration with change of gears leads to something from the back of the transmission to the back of the van. No vibration while on the jackstand usually means that it is not a tire issue. No vibration on the jack stand noiticed would be consistent with one bearing cup going bad in a u-joint as the wheels place no load on the joint while on a stand leaving it centered and not vibrating. 1st, look at the cup seals for rust. If none observed, you can sometimes find it by applying the parking brake and manually twisting the driveline looking at the joint where it goes into the cups and you may see movement. Any movment at the cups is from a failed bearing. Sometimes you just have to pull the driveshaft to find out. You can take the rear loose first and feel it for free movement without risking loss of fluid from the back of the transmission. if the front needs to be checked, you may lose some transmission fluid when you pull it.

 

[buba_zanetti]

Sweet, lots of good info on here in a short time. Thanks everyone for all the wisdon. I'm leaning toward the U-joint myself. The visible shaking of the driveline is at the transmission output and the front of the driveshaft. Of course, the van is also reasting on jackstands under the rear axle.
By "checking the tires" I looked for an obvious "clean spot" where a wheel weight would have come off recently, and a thorough look at the treads. The tires are due for rotation. I will just rotate them and see if it goes away magically (tires only 5k-6k miles old). If it doesn't go away, I will go at the U-joints. One of my racing buddies works at a shop and can do U-joints and shaft balance for about $100, though if the U-joint(s) is/are obviously shot, I'm sure I can do them in the driveway. When I looked at them the other day they actually looked surprisingly clean where the rubber seals were, but I was mainly looking for the grease fitting (which they did not have), not looking truly inspecting them.
I just need the van solid to sell, as my '95 B2500 replacement is road-ready (I just need to swap my tow receiver over to it and do the legal on it). Will post up results when I can get under there again (probably not until Monday).
b-z-

 

[John Wood]

U-joints are not really that expensive, and at 133k miles with sealed units (i.e. no grease fittings), I concur with the rest of the folks that replacing those would likely take care of the problem. You've probably got ridges where the needle rollers ride which will cause the vibration.

 

[buba_zanetti]

Woo hoo! $8.99 each for the Duralast budget-brand U-joints and the 'ol gal is humming along again. I could not find any slop in the U-joints while the driveshaft was connected, but as soon as I dropped the rear joint I could tell; the rear had the "notch" feeling in one plane of motion, and the front was completely frozen in one plane of motion. It was so tight that the weight of the driveshaft could be supported, just hanging from the frozen front U-joint.
It took me quite awhile to get a rhythm down and get the joints changed without a vise, but I managed. I'm glad it was something easy on the pocket book.
Thanks for the adivce everyone.
b-z-

 

[68RT]

Nice to hear that the problem is solved.