The little muck bucket carts (about $55) would help with moving things within the confined spaces in the barn and then with just a decent piece of wood you could ramp right in to your spreader/truck/whatever. You could even create a simple pully system so that you can remain on the ground and hoist the bucket up.
I like the TSC 4 wheel dumping cart idea for you. You could hand wheel it to each stall and muck right into it, then attach dumping cart cart to 4 wheeler. I also like the electric muck bucket lift idea for your current manure spreader set up. FarmTek has some easy electric winches that you could hang over your narrow aisle and lift a full muck bucket to dump into the spreader.
My contribution as a 66 year old daily 4 stall cleaner: Dump a coffee can amount of bagged stall pellets per each stall on the pee spots before re-covering with clean bedding. The pellets will soak up the urine fast, and keep it from spreading to more bedding. It forms a flat, dense pancake of very heavy urine soaked sawdust (hence the winch for lifting muck buckets) , but the daily clean-out amount is much less volume per stall (less forkfuls to lift) than before I started using pellets in this way, along with shavings. Hopefully your horses pee in approx. the same spots. Works even better with straw.
Try setting your stalls up like I do: bare in front corner for eating off ground, ~4" in center for pee spots (with pellets under), and shavings banked along one wall and corner for a “bed” that is about a foot deep - extra shavings high in the corner. They will sleep on their ‘bed’ as horses like to have their feet lower when lying down, and you have less deep bedded area to clean. The banked shavings in the corner mean you have to add new shavings less often, just pull down from the corner. Pawers would be a problem, though. I only had one, once.
Also, pick manure piles into a corner in the morning and at bed check. It is easier to clean out the corner than sift through a large bedded stall for scattered piles.
Ask your vet to make you up some liquid DMSO/Dex (50/50) in a dauber bottle and apply it over your back pain spots - add Aloe Vera over it if it prickles. Will help immediately with any inflammation pressing on nerves and on pulled muscles or ligaments. I use it morning and evening for 4 days only, then a rest. Never had to go beyond that.
Good luck to you.
