vibration after Jasper transmission install

1984 XJ Pioneer. Stock except for 3.4l engine replacement of 2.8l boat anchor. No vibration issues after engine install. Transmission is vehicle's original A904 Torqueflight remanned by Jasper as they didn't have one in stock. When vehicle accelerates thru 40 mph the whole back end starts vibrating significantly. Feels like rear driveshaft but I am no mechanic. The vibration reduces getting up to 70 mph but is still there. Interstate mpg has dropped from 17.5 mpg to 13.5 mpg. I don't want to think Jasper made a lemon especially as this transmission had no vibration issues before being remanned by Jasper. Any idea what installation mistake or other issue might cause this?
Thanks!
David

How is your trans mount?

2wd?

Do not assume it is the transmission. Check the drive shaft and u-joints first. Swap some tires to see if a rear tire belt broke inside the tire, check for loose lug nuts on the tires, simple obvious stuff first. Check the flex plate bolts, see if any are loose, and the Transmission mount.

Oops! Thanks! 1984 XJ is 4WD and a 4 door. I will crawl under the Cherokee this weekend and look for anything obvious.

Why did you rebuild the Tranny?

It dumped all of its fluid on I-295S almost all at once. It also had about 230,000 on it.

Pinion bearing? Now that there's a real engine and tranny...I've had u-joints feel like that, too. Hopefully, it's something cheap and easy to fix. Let us know what you find.

obxxj said:

It dumped all of its fluid on I-295S almost all at once. It also had about 230,000 on it.

Click to expand...

What dumped it? A line break, seal, TC seal????

I would check by jacking up the rear end put it on jack stands and let the XJ idle in gear see if any thing seems out of wack. Jusr make sure you have good jack stands!!!

Thanks everyone! All this is on temporay hold as I crashed my motorcycle and dislocated my shoulder and compound fractured my right arm. Bike came out better than I did but it'll be awhile before I'm rolling about under the jeep.