I just picked up my 90 2 door, manual, 4.0, 4x4 (you have no idea how hard that combo is to find) for real cheap. The guy I bought it from could not get it to pass smog, so I knew there would be some problems. On the drive home I noticed that under power and only in 1st gear I get a rattle or knock that seems to be coming from the transmission tunnel, but I cannot be sure. It only lasts a short time and only at about 3000-3500 RPMs. When I got it home i did a basic tune up and redid the vacuum lines, it passed smog with flying colors. Now that I know its legal time to start fixing the little problems. The rattle is easily replicated, but like I said only occurs under heavy acceleration. I looked for loose exhaust or linkage, but did not find any. I did not check the motor mounts yet (it got too dark), but the motor does not "buck" when reved. I know it has an exhaust leak, I am assuming it is the manifold (not sure if that could be connected). I know its not the fan shroud, it doesn't have one. Any ideas on what else to look for?

PS Is there an easy way to open the lift gate from the inside, I don't have they key.

 

Skid plate?
Packing in muffler/cat(s)?
Exhaust hanger?
Loose nut somewhere? (ha)

 

Check where the exhaust pipe goes above the trans/t-case crossmember. Failing mounts, both engine and trans, can cause the pipe to contact the crossmmber under acceleration.

 

So, note to self, stop working on jeeps before 11PM, the bent fan blades were hitting the crossbar the shoud mounts to. How much clearance should be there?

 

Your motor mounts have to be shot or a motor mount bracket loose on the engine.

 

yea the bushings are shot, how hard is to change the drivers side bracket, it seems a ***** to get to

 

I have the exact same problem with mine!! I now have a squeak at certain rpms, let off or accelerate it goes away!

 

Really ought to replace both motor mounts and the trans mount at once. Not a bad job. Easier than it looks.