... still getting a weird low rumble/vibration while accelerating starting at about 40 mph. I just read through the following thread and despite having a lot of good info about the rear driveline, the OP never actually fixed his similar issue:

1999 SR5 Driveline Vibrations

Since I'm still new to the 4x4 world and the fact that I had some pretty decent drops and hits the last trail day, I'm wondering if the tires may be unbalanced? I do have a spare driveshaft, so I may just try that guy out for shits'n'giggles this weekend. There's still a pretty decent angle to the front axles even with the new diff drop, so I'm also thinking that I may need to remove the strut spacers and deal with the wheel well mod for the 33's. The front axle angles are looking pretty dang good now that the suspension has settled after coming off of the stands last night, so I'm wondering if it may be something else around the front diff that I should be looking at? We shall see...