It depends on the leak source. As a first step, place a piece of cardboard underneath the rear differential to determine the location of the leak. It could be the axle pinion seal on the front of the differential - just behind where the driveshaft u-joint is located.
If this seal is leaking, then this is a fairly simple fix. Remove the driveshaft (be sure to place marks on the u-joint and yoke to index the shaft in the same position during the reinstallation). Next, remove the differential center yoke (where the driveshaft rear u-joint is located). Once the yoke is removed, then you can use a seal puller to remove the old pinion seal.
Clean the seal mating surfaces on the differential, then install the new seal (use a seal install tool) being certain of the seal installation direction. The rest of the installation is the reverse of the removal steps.

Try the many on-line auto parts sources, such as; www.autopartswarehouse.com, www.partsgeek.com and www.rockauto.com for the replacement parts. For example, they (RockAuto) list the rear differential pinion seal from around $6.20 for your vehicle.

According to a basic lookup in All data on transmission overhaul, there is a retainer that comes off with a snap ring pliers that is described as " the output shaft to input carrier retainer" and after removing that retainer everything comes out without an issue. Remove the input carrier assembly and the bearing assembly. Remove the input internal gear and the reaction carrier shaft assembly. Remove the thrust washer and the reaction sun shell.

You may find some needed part numbers here in this article which is a TSB for Hummer tranny issue. http://www.cadillacfaq.com/.../00-07-30-022c.pdf

It sounds like a heat shield for the catalytic converter or the honey comb inside the cat. If it is the cat, this vehicle is still under the warranty for 8 years or 80 K miles.

Unfortunately, the W0133 axle lacks information thus im unable to make any differentiation.

My suggestion would be to just measure your existing axle and compare it to the specs of the EMP801505

Brand New for me chief.

transmission could have internal problem or the the shift linkage is out of place. You have to tow the vehicle to a shop for proper diagnostic.

You should check the condition of your axle shaft bearing, might be dry/worn/damaged.

You have a bad instrument panel cluster. Call a local GM dealer and ask if they can "goodwill" it. use those words. Secret campaign as these clusters are dropping like flies. be nice though, they are not obligated to do anything for you. A new cluster installed would normally cost over a grand.

Had my cluster rebuilt for abt $150. Check on eBay. Several guys do it. Mine works great now.

I had mine repaired for $179. Gauges and lite diode. Sent unit to Minnesota and had it back in 5 days. I believe it was D & D electronics.

Usually in this case possibly the parking pawl gets broken, what is also possibly it sounds like your shift linkage possibly broke or what they might have done to move the vehicle was disconnect the shift cable or linkage if that is what your vehciel uses and manually moved it into neutral to tow the truck.

And you have not mentioned if any warning lights are on? Any warning messages? Any service light? You must attach a code reader and read the fault codes.

Make sure you are not holding or leaning on the gear selector . this sometimes makes the transmission think you aer trying to shift it.