Transmission / Xcase issue

I recently got a 1981 CJ7 with an AMC 304 and what I'm fairly sure is a Dana 300 transfer case. I haven't gotten under it to find out what model transmission yet.
The problem(s) are that sometimes in 2nd gear and sometimes in 3rd gear, I get a whirring/whining noise and vibration under load (acceleration or uphill). Also, it seems to chug over 40mph in 4th gear.
Could the noise/vibration be an transmission issue? Bad bearings or gears?
It doesn't bind or skip on pavement, but I'm not 100% sure the transfer case is in 2H (seems finicky). I don't know why else it would struggle to do more than 40mph otherwise. The engine revs like normal when not in gear.
Anyone else seen/heard/dealt with something like this? I know that's a lot of not-very-specific info - I just need some direction on what to check into first.

 

Do you have a 3sp, 4sp, or 5sp tranny? Looks like you may have a 4speed based on you post.

Looks like you have bigger than normal tires. The 81 CJ7 was likely geared with 2.72 or 3.07 gears in the axles. The combination of big tires and high gearing is the likely culprit of the 40mph lugging. Consider installing 4.10 gears.

The whirring/whining is likely a bearing. Hard to isolate in 2 or 3 gear. Could be input/output bearing in tranny, input/output bearing in Tcase, pinion bearings, diff bearings. driveshaft bearings. If you have a 4sp and especially a 5 speed tranny the drive line vibration is likely due to the steep angle of the drive shaft caused by the lift kit exacerbated by worn uJoints.

I would start by replacing all of your uJoints in the driveshafts. Get the non-greasable ones as they are stronger and require no maintenance untill they wear out again. This might fix both the vibration and the squeal and it is a cheap fix. If you still have the vibration next would be Tcase drop (my least favorites) and a double cardigan (CV) driveshaft (what I did). If you still squealing you are just going to have to hunt it down. I had a squeal and it turned out it was the output shaft bearing on my tranny.

Read this post for figuring out what tranny you have:

https://www.jeeps.net/threads/transmission-identification.2146/

 

The transmission is a 4sp - likely a T-176 based on the straight shaft and shift pattern (from what I've read). I haven't ID'ed the differential/ratio yet but I suspect its not OEM.
I should have mentioned, it was driving fine 50-60mph with no problems before it got cold out. Not sure if that makes a difference or is coincidental.
In any case, I'll look into new U-joints and replacing the bearings as a starting point. Thanks so much for the direction!

 

Do you have a way to pick the Jeep up? I have put Jeeps on stands before and run them through gears. You can also have someone walk around the Jeep to pinpoint where the sound is coming from. Be very careful doing this though, because if it fell of off the stands it could be bad.

~Jeff

 

Welcome to the forum GRNID3R.
Your in good hands with OFFGRID and Southeast Overland.

 

Resolution (sort of)... started with the transmission and checked the fluid level. It must have been nearly dry because it took 3 pints. What's weird is that I haven't seen a single spot under it in it the month I've had it so far. I'll just have to keep an eye on it and see if it needs new seals and gaskets.
So the whine is gone, and shifting actually feels better. The sluggishness in 4th gear is gone as well - maybe it was really binding and working the motor in high gear.
Thanks for the help and direction guys!