It's either the sender unit or the gauge itself. If you have a multimeter you could test the sender unit's resistance when the engine is cold and during the warm-up. If the resistance suddenly jumps, replace the sender. Otherwise it is the gauge. I don't recall a gauge ever needing to be...
For ball joints one should always pop for the Genuine Toyota Part. I know they are expensive; however, there have been too many ball joint failures with aftermarket ones over the last 15-20 years.
You can do better than $500, but not much. I bought one and paid $430 including shipping. Here is the info:
Part Number
Part Name
Price
Quantity
Total
43430-04020
Axle Assembly
$428.45
1
$428.45
Subtotal:
$428.45
Shipping (Free Ground Shipping):
$0.00
Handling:
$3.00
Total:
$431.45
For all engine-related parts, I recommend only Genuine Toyota Parts (but you don't have to buy them at a Toyota dealer. For timing belts, I thing "aircabinman" on eBay is your best bet.