Sounds like problems with the low reverse servo engagement. The cause could be a variety of things including fluid level, solenoid pack, valve body gasket, servo, etc.
Interestingly enough, I have an update to this related issue: Ford Mustang Forum - View Single Post - Transmission started to whine in 2nd gear?
After 60k miles on a new replacement transmission, my car started having problems getting into reverse, similar to yours.
I pulled the pan to start checking things out.
So far, I found that the low reverse servo housing was not assembled properly at the factory. The 2 halves of the housing are supposed to rotate together until it can click and lock in place. Mine wasn't locked, and the whole thing popped apart when I unbolted it.
Looks like it may have rotated over time until it came apart and made the servo operation a problem.
First time that I've heard of this cause. Most other ones are related to solenoid packs, dirty valve body, and eventually, burned and broken bands.
Don't keep driving with the problem. After a while, the reverse band breaks and will require a full tear down of the transmission to replace.
How many miles do you have on your car?
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