2006 E150 4.6L

Moved my van and found small puddle of heavy oil where my left rear tire was. Looked underneath and inboard side of tire was covered with oil. I assume the axle seal on that side is leaking. Will pull tire and rotor tomorrow.
Is changing seal DIY job? Should seal on other side be changed at same time? Any other tips or things to look out for?

 

If it is a 150 series it probably has a "c clip" rear axle. You have to pull the axles to change the seal.

 

You can get by doing just the leaky one, no it isn't a DIY unless you have certain skills, it's best to have it done, it's a long drawn out process involving cover plate removal, gear oil change, may as well do the brakes while you're at it and repack the bearings.
I've pulled C-clip rear axles and rebuilt a rear end once, even have replaced the seals on my 99 E-350 I currently drive, no everyone can do it, and not everyone should try it, depends on your skill level, cause it leaves room to make a mess.