2000 S-10 Vibrations

I have a 2000 S-10 2wd extended cab with stock suspension, stock tires (Tigerpaw AWP's with about 25k on them) and 98k miles on the truck -- it vibrates at about 10-12 mph, 35 through 40 mph, at aboy 60mph and at about 70mph. The vibration at the low end almost feels like a bump on a tire, but it's not. The vibration does not come through the steering wheel, but I feel it in the floor boards and it makes the jump seat vibrate.

Here's what I've done so far:
1) Had all 4 tires balanced, no change.
2) Rotated the tires front to back on both sides, with no change.
3) Checked the shocks - no visible signs of leaking and they are still stabilizing the rear end when I bounce the back of the truck.
4) Checked the drive shaft (a 2 part with a carrier bearing in the middle) and there is no play in any of the u-joints.
5) Checked the entire drive shaft for visible dents or missing weights -- everything appeared to be normal.
6) There is a very slight buildup of grease or lube at the back of the tranny, but it isn't fresh (more dirt than lube).
7) I noticed a very slight amount of play in the spines on the drive shaft behind the carrier bearing, but that went away when I put grease in the grease fitting.
8) Jacked up the back wheels, one at a time and lifted the tire with a bar and heard a click from the wheel area on both sides (the same on both sides).
9) Checked the rear leaf springs and they appear to be OK, although when looking at the back of the truck, the driver's side appears to set a bit lower than the passenger side -- ever since I bought the truck (at 22K miles), this has been the case.

At about 40K, the rear axle bearings on the driver's side went and the dealer redid the bearings on both sides.

I thought it might be a broken band on a tire, but when I rotated front to back, I should have felt a change in the vibration.

I use the truck as a daily commuter truck, very rarely hauling more than groceries or maybe a sheet of plywood or a few sticks of lumber.

Any suggestions as to how to narrow the problem more without taking it to a shop and letting them charge me through through the wazoo?

Thanks

I had a similiar problem with a truck I used to own, found the rear driveshaft/front u-joint binding at it's widest range of motion and had to remove the driveshaft & mid bearing to find it.