Massive transfer case shake when under light load

Hey guys! I'm utilizing you as a last resort because I've got a very weird problem going on that I've never seen before. I was towing my boat a few days ago (~1500 lbs) and didn't notice any problems. Used 4hi on the boat ramp, and it did seem to shift back into 2wd kinda "hard" but nothing more crazy than usual.

Anyways, I drop off the boat at home and go to get some food and in the parking lot between 5-10 mph the entire truck shakes, under the seat and bad enough to rattle your teeth. It feels like the wheels are literally squares. So today I jack up the rear end and run it and the vibes are even worse! The driveshaft, rear diff, carrier bearing, all look fine and no vibes when I hold my hand to them. But the tail end of the transfer case is shaking violently! So does the exhaust, and the trans and engine but to a lesser degree. I can't tell if it's the transfer case causing it, or if it just looks the worst because it's at the end of the line. All mounts I can see look okay.

It's worth noting that under load there are no vibes. Heavy acceleration from stop - no vibes at all. The less load or lighter acceleration the worse the vibes. But vibes are present in all gears, reverse, and 4hi although not as bad in 4hi. Vibes are present from 5-10 mph mostly, and seem worse when turning.

Has anyone had this happen before? Any ideas before I let this thing disappear to a transmission shop for a few months? Thank you!

Edit: I should also add, this happens on acceleration or deceleration. The common factor is light load. So heavy accelerate or brake, no vibes. Light acceleration or brake, bad vibes.

 

My first guess is a universal joint .
You cant tell if a universal joint is seized up by looking at it .
The only way to tell if a u joint is bad is by removing the driveshaft .
Have you been greasing the driveshaft at every oil change ?
https://www.tacomaworld.com/threads/vibration-diagnostics-for-all-tacomas-write-up.520607/

 

I have greased them regularly, but I'll go give it a shot. The shaft seems to spin freely when I'm underneath it and it's in drive. Thank you!

 

OK great . Glad to hear you grease the u-joints on a constant basis .
With the truck jacked up , you still get the vibrations ?
Have you ever taken your slip yoke apart to lubricate the splines ?
Check to see if somehow a couple rocks managed to slip their way into the transmission mount .

 

I just went in there and replaced the forward most u joint since I keep extras on hand and it turns out one of the cups was seized. Crazy, considering I last greased them probably 5,000 miles ago. The problem is solved now. Thank you so much for your help! I've never had a u joint fail like that before.

 

You are hired . I need a guy like you hanging around my shop

Thats about the quickest U-joint change on Tacoma World lol .
Glad you got it fixed up and soooo quickly