[hatzie]

GM didn't put 4 cylinder engines in the 7th or 8th gen cars. You should have a 4T65E-HD behind the LS4 V8. If you have the lighter duty 4T65E transmission out of a V6 it will not like the V8 torque and it will not be geared properly either.

Whether it's worth fixing depends on where you live and whether it's a rust bucket with a trashed interior and covered in dents or it's in decent shape other than the transmission.
The LS4 V8 Impala SS and her sisters are getting less and less common as she marches rapidly toward the end of her 2nd decade on the road. If it's not a rusty turd it's probably worth fixing. Make sure you have the title in your name before you dive into it.
Decent examples of the LS4 V8 Pontiac Grand Prix GXP, Monte Carlo SS, Impala SS, and Buick Lacrosse Super are not going down in price.

The 2004 & 2005 Impala SS with the supercharged Series III 3800 V6 and the 1997-2003 Pontiac GTP and even some of the 1990's Buick models with the supercharged Series II 3800 have started climbing back up in price.

The 4.6L Northstar V8 Pontiac Bonneville GXP is fairly stable in price too. Eventually they'll go up but not as fast, as soon, or as high because of the engine reputation.

 

[hatzie]

Check pressure control solenoid

You'll have to drop the LH end of the engine cradle and pull the LH half shaft to access the valve body.
If you're down to the valve body replace both shift solenoids, Torque Converter Clutch solenoid, Pressure Control Solenoid, and Pressure switch manifold.

Here's the common problems page from TEP.
Triple Edge Performance

Slow/No reverse engagement is fairly straightforward.

Triple Edge Performance

tripleedgeperformance.com

Whining... I would check the accessory drive idlers, tensioner, alternator, and power steering pump.
If it's coming from the transmission take a look here.

Triple Edge Performance

tripleedgeperformance.com