6 month old puppy with leg weakness?

Hello,

I haven't done a forum post before. So I have a 6 month old full rottie pup ( the 11th was her birthday).
She hasn't been spayed yet, tail has not been docked and due claws have not been removed. We have had her since 8 weeks old. She is my husband DREAM dog, she loves our other dog and our cats, and has the best temperament.

I took her to the vet for a possible UTI (which she slighty had) however they are more concerned with her walk at this point, she is basically acting like a 10year old. Showing signs of hip dysplaysia.. She walks with a gait, she often sits and moves like a frog. When she runs her legs are straight and moving at one time. Bunny hops, difficulty getting up. I mean she really acts like her hind legs are weak and not working. The vet took x-rays and they are CLEAN, her hips are PERFECTLY positioned and joints are covered.
The vet recommends we take her to have an MRI checking for nerve damage however I've seen a few posts that she could just be going through some growing stage and its really hard on her, she whines some while biting at her back legs like something see hurting her. The vet also checked for ticks but said there were none.

So my question to you all is this. Has anyone else had hind leg problems with their rotties as they were growing up? Should I wait before I get overly concerned, I mean its all I notice now of course! and my husband is completely devastated that we might have to put her down or not be able to care for her. Honestly I can not afford an MRI (our area is about 2300 cost wise) and I don't want to get it done to early if it could be something she might just grow out of.