My truck has lost 3rd gear and drive. The transmission is a GM 4L60E. In qualifying a reman vendor, who sells the good stuff and who sells the junk.

I have installed a number of Jasper transmissions, engines and differentials and rarely have trouble with them. They have a good warranty. Be sure there is no underlying factor that might have caused or contributed. >1996 models have a MAF (mass air flow) sensor that must be clean and functioning. And clean out the cooler in the radiator, the auxiliary cooler if it has one, and the lines, or any bad fluid/metal particles in there will be sucked into the new transmission.

Might be in transmission limp mode, try disconnecting battery for couple of minuets, then reconnect and test drive.

You can get a new 4l60e at your local dealer. Rebuilt, could be hit or miss at some shops. A lot of times the transmission case is bad. There is a steel sprag that is keyed into the alum. case and the fit gets worn out after a lot of miles or hard use.
Then for a temporary fix some rebuilders put in a bandaid part called "case saver". It will last the warranty then fail.

A new tranny would my choice over rebuilt.

Go with the GM remanfactured transmission. 36 month/100,000 mile warranty. Latest refinements and updates are installed, anything that does not meet OEM standards is replaced. I sold these for many years and had a couple of cars and trucks they were installed in. Last one I had replaced was a 4L60E, at 80k. When my daughter traded the truck off at 160k it functioned like it should and no signs of fluid seepage or leakage, fluid was still nice and red.

From the last information I have when I retired (2017)
Parts and labor warranty when installed by a GM dealer or qualified installing independent service center. Parts and labor warranty when sold over the counter (you buy it and take it for installation)and repaired by a GM dealer or qualified installing independent service center.

That is for passenger and light truck uses. Medium duty (C-40-80, class A motor home, taxi and police use is 18 months/100,000 miles. The sold part must be correct by parts catalog usage, for example you have a 305 engine in a pickup and replace it with a 350, the warranty then becomes 12 month/12k miles as that is considered a non-catalog usage, in other words it would fit but was not originally intended for that particular vehicle. Don't let the dealer rebuild or repair your transmission, the warranty is only 12 months/12k when that is done through a dealership unless it is something very minor like a shift solenoid or similar.

You can usually "deal" on that type of part. GM has different discounts depending on dealer volume (a.k.a truckload, 53 pieces) and sometimes rebates available. And sometimes there were rebates available when the dealer installs them, last time I remember it was like $200 back. If one dealer offers the rebate,shop around, it's not an individual thing if it's a GM corporate offer. Go for the best price, it's all the same product. And if the dealer says he has to order it from another dealer ask if that dealer is offering free freight. Many of them are offering that to other dealers in a specific geographic area. No sense to pay freight if not necessary.

If you were closer I would set you up with one of the dealers I used to work for.

Wrecking yards are full to the fence with 4L60E transmissions. Get one for $150 and spend an afternoon hoisting it up.

^^^ This ^^^

While not for everyone, a rebuild kit runs about $80 or so. Takes a weekend or so if you are adventurous.

I would speculate that it ran hot or was low on fluid. This lowers the pressure on the clutch packs and fries them over time. Valve body should be fine. The best way to see if it is the pump would be to put it in reverse. The pump runs 90 or so PSI in the forward gears, but over 140 PSI in reverse.

Do a scan for trouble codes first thing.