How would i know?
 

What would be some signs that a rear end is bad or going bad or needs replacement?

Listen for a grinding sound or a metal on metal sound. That might tell you that it either needs to be replaced or you need more grease in your rear-end.
.
.
.
.
no pun intended.....LOL

haha...lmao...that was one of those things that was like 1...2...and ok...lol...

o ok...see only b/c it seems like at a lower speed...like pulling out of a spot mostly...i get like a shaking sound or feeling....almost like the wheels are slipping a lil...but idk...the tires arent the newest and do need chnaging...but its a weird sound or vibration im getting...and it isnt consistant...

Automatic or manual transmission?

If auto, could be low on fluid causing the transmission to slip a little on take-off.

it doesnt feel like the tranny is slipping....but it seems more when im making slower turns or pulling out of parking spots...also happen backing up to parrallel park...idk if its just that the tires are down to low tread and its cold out...so the tires are slipping or hopping a little bit....but doesnt happen all the time...

I don't know about your truck but mine feels like it has a locking differential (I know it's not), the tires always spin at the same speed...It's real dangerous pulling out onto a slick road...but when I pull into tight spots or making a U turn, I can hear the sound of my inner tire slipping on the pavement, kind of makes the sound your describing.

for the time it takes, you might as well jack up the rear end and pull the cover and have a look inside. broken teeth,milky oil,and excessive slop are all bad signs.:)

^^What he said and also look for metal shavings in the bottom of the transfer case. That's NEVER a good sign.

Also... Please know...
 

That early '04's had rear end problems. Not so much what you are speaking of, mostly a high pitch noise that just gets worse. The solution was to just put in a heavier gear oil to silence the noise until it got out of warranty. I wonder if your rear end is affected?

My brother runs a Mercury/Lincoln Dealership and it pained him to tell me this.

lift the rear end off the ground so the tires are only 2 inches off make sure you block it up.have a friend help you.chock block the front tires.have youre friend start the truck and put it in gear and let the back tires rotate.slowley.listen for sounds in the diff also lison for noise in the drive shaft.put youre hand on the rear diff and see if you can feel any grinding or knocking.if you dont feel safe in doing this take it somewhere you trust.remeber anyone can r/r pinion gear and carerior but you must check the clearance.