How hard is it to change existing rearend to include posi or tloc rearend cost and time as well?
or have pro do it?

Traction Lok can be added to the 7.5 gear, parts would cost in the area of $300-325, labor is approximately 4-4.5 hours. Unless you have experience with differential repair, a pro is definitely recommended.

You can get a new FRPP 7.5" Traction Lok Differential kit for under $200.00 if you search hard enough. Searching for FRPP Part #
M-4204-C75
and
M4204C75

I just found prices ranging from $169.00 to $239.99 (www.gefracing.com had the lowest, but personally have never bought from them).

Traction Lok, BF Goodrich KDWS Tires, Magnaflow Dual Exhaust, and a few other items were installed on my 2004 3.9L V6 5-Speed before it had even 100 miles on it. The old "One Wheel Peel" was much worse from a dead stop on this car than on similarly equipped '94 & '98 V6 5-Speed Mustangs I had before ... 3.27 compared to 2.73 rear gears and 43 more HP did not help the situation !! The 2005 having slightly lower 3.31 gears and 17 more HP (than my 2004) should be even worse yet, at least ways with the manual transmission.

T-Lok installation cost me only $50.00 ... friend in my car club sets up rear-ends as a hobby and I brought axle assembly to him (I had it out for painting and it is much easier to do on a bench). But if someone were to do it for you "in the car", I can't see it costing more than $150-250.00 in labor.