Strong vibration from rear while in reverse

[Towncivilian]

Whenever I'm in reverse, there's a strong vibration coming from the rear of the vehicle every few seconds. Transmission fluid/filter changed at 120,340 miles with Genuine Nissan ATF & Beck/Arnley filter/gasket kit, but the fluid smells a little burnt (still red, though) and somewhat overfull so I'm thinking of doing a drain and refill using Valvoline Maxlife ATF to clear that up. Rear differential fluid has probably never been changed; will change that as soon as I get a new drain plug (both plugs are stuck, several applications of PB Blaster, a hammer, and a 25" breaker bar should take care of that though), using Valvoline 80W-90 gear oil. Something I'm missing, or is this just normal behavior? I believe this also occurred before the fluid change too. Vehicle currently has 121,092 miles.

[Towncivilian]

BITOG says bad u-joint or drive train mount, and says that if there is a slip joint to repack it with grease, or if the u-joints have zerk fittings to grease them. Are either of those suggestions applicable?