A picture of the set-up may be helpful to see. In general, regardless of cause, if the high-lip is causing issues, I’d get something lower so it’s easier. Also make sure that the flooring both in and out of the cage is rabbit-appropriate (i.e., not slippy) to maximize movement/jump ease.

Was the local vet rabbit-experienced? Was an x-ray taken? It would be difficult to conclusive rule out any thing and say “nothing wrong” without an actual x-ray. For example, unless it’s unfortunately obvious based on angle/skin breaching, it is often not clear whether a rabbit has sprain or broken a limb without an x-ray. It could also be something higher up (e.g., spine), rather than the legs themselves. Jumping requires more than just leg movement — it is a full body endeavor.

Could she clear it just fine before? Since she is 5, she could have a little bit of arthritis developing.

Hind leg weakness can also be a symptom of E. cuniculi. Worth considering as a possible cause.

As Azerane say, e cuniculi does come to mind when hind end weakness is discussed. EC is an intracellular parasite that buns often get from their mum while they’re still in the womb. It often lies dormant for many years, sometimes the entire life.

It’s treatable with a dewormer, fenbendazole. Its sold under various brand names, the most common one being Panacure.

If the dewormer doesn’t help it could be as LBJ suggested, arthritis.

I previously had a bun approaching 5 who developed arthritis fairly suddenly and it presented in a persistant hing leg limp and hesitancy to move too much and jump. Daily pain meds help her a lot to move more often and even frolic but her limp never really went away.

As Glenn says, you could ask your vet to try some daily meloxicam. It’s an anti-inflammatory painkiller that helps both with joint pain and the arthritis that’s causing the pain. You should see improvement in about a week, or sooner.

After a few weeks you can as a rule start decreasing the dose until you find the smallest effective maintenance dose. This is a standard treatment for aging dogs, but its becoming more and more common for rabbits as well -most likely because our pet buns live longer nowadays.