What could this vibration be?
 

In my suburban ('88 EFI 350 TH400, I think, does TH400 have OD?)

when i get up around 50+ mph i get a horrible vibration throughout the undercarraige, Im hoping its not tranny... any ideas?

it goes away when i back off the throttle and get down to around 45 mph
http://www.urbandev.net/newsuburban.jpg
$300 pulled from a junk yard :x
runs good...nice body...nice interior...

Sounds more like an alignment/wheel issue to me. But someone qualified really should back me up on that, before you do anything...

Could be an alignment/death-wobble resonance, maybe tires or a driveshaft. Does the steering wheel shake or jerk side-to-side?

Sometimes it helps to have a friend drive behind you to provide visual clues.

the first thing that comes to mind is a driveshaft carrier bearing or U-joint. Does the vibration increase or decrease with speed above 45mph (where you say it quietens down)?

increase, it just gets worse when i speed up

steering doesnt shake

That would be my call. I'm guessing it's the U-joint. Not a huge deal, but I'd get it fixed before it breaks.

Well.. let's start with the least expensive route first...
Crawl underneath, and see if possibly yo might have thrown a balance weight or weights on the driveshaft look all the way down from the tranny to the rear end.

If no luck, try grabbing the driveshaft at each U-joint.... if they're good, you shouldn't get much play out of them. I would think that if the culprit is a U-joint that it ought to be pretty sloppy to cause the vibration you describe. If it does end up being a U-joint, replace ALL of them, along with the carrier bearing too if you have one.

Still vibrates?

Since the steering doesn't chake, then your front end ought to be ok. I would check for a bent wheel, or anything that looks bent or obvious in and around the rear wheels.

I really think that you're gonna find oen or more bad U-Joints though, and/or a bad carrier bearing...

Lemme know what you find.... and Good luck!

I can't really add anything to what's been said other than to say that the transmission is going to be the 700R4 not the TH400. The TH400 has no overdrive and by 88 i believe that GM had completely phased out the TH400 in light trucks

cj2112, the TH400 doesn't have one stock, but if it's EFI it might actually be a 4L80E, which does have OD and is essentially a TH400 with overdrive and electronics added. There's also overdrive kits available for the TH400 and if its coming out of a scrap yard that could be the case.

I'm going to put my vote in for U-joints as well, but it could be the rear driveshaft too. Check the ends of the driveshaft for play, if they don't move check the wheels. If the wheels are okay you may want to get the driveshaft re-balanced and see if that helps.

yes it could also be the 4L80E but I don't believe those were available in 1988, although i could be wrong

Right, I had to bust out the books to verify this now.

You're right, the nomenclature change was actually in 1990. For some reason I thought 88. You've also got it right that the TH400 was phased out of light truck duty in the early 80's (this I already knew, but for the sake of clarifying).

Which means it's going to be a 700r4 and I've (re)learned something here. Although it's still possible that it's a TH400 with an OD kit (I've seen a lot of those swapped in where a 700 used to be due to their durability) the OP didn't mention any modifications, so we'll assume stock and roll with it.

K ill crawl under there and shake some shit around when i get home, there's also a dent in the driveline...so i think ill replace the shaft as well as the u-joints

A dent often means a bend, even if slight. Doesn't take much to cause a bad shake given the shaft spins ~3-4 times as fast as the tires.

Don't discount the steering. The '88 Suburban shipped with a steering stabilizer. If it's failed, the steering might feel tight, but the ride at 40+ mph will rattle your fillings loose.

700R4 sounds right for the trans.