I think the forward clutches might be welded together in my sons 700R4.
He was stuck the other day and was rocking the truck back and forth from drive to reverse trying to get out. The operationg temp got up to about 220*.
Then some smoke rolled out from under the truck and we could smell trans fluid.
After we pulled the truck out we noticed it felt like we were pulling an anchor when in reverse.
Now low gear feels sluggish and with the trans in reverse the truck won't hardly move. Like the brakes are applied. I checked all the brakes.
Any ideas?

 

Definitely sounds like you did some damage to the transmission. It is difficult to diagnose it over the internet with the information you have given.

How does it operate after you have cleaned it up and checked all fluids? Does the transmission fluid look/smell burnt?

 

Sounds like it's fragged. Remove the trans pan and look for chunks/excessive clutch disk material. If none, try a filter/fluid change. If that does not work then remove trans for a rebuild.

 

After you pull the pan, and IF you do find lots of debris, know this.
Pulling the tranny yourself will save you half or more in getting it built by the shop.
Copper flakes in the pan indicate a worn front pump babbit bearing.

I called 2 shops in 2 different towns, $1200-1400 if they pull it. $550-600 if I do !
One shop would not allow the customer to pull the tranny without voiding the warranty, Butt's !!

Make sure to leave the torque converter attached to the trans, after removal inspect the shaft and the front pump bearing.
Make sure to get a new front pump and torque converter if the bearing is worn badly, this is misdiagnosed as a bad torque converter when Both are in need of replacement.
[The old shake when TCC is applied, that stops upon applying brake while maintaining fuel]

A new converter and front pump is good insurance when doing a build.
Also a "Corvette Kit" with more metal parts and better clutches etc is readily available from Jegs, Summit etc.
This kit will prolong the life of a heavy usage tranny.

After the build, make sure to engage BOTH torque cover tangs into the front pump before bolting up, it is a 2 step drop into the front pump, if the tranny don't go to the motor all the way easily, you didn't.

Clean the debris from the cooler and lines or damage the new rebuild.

If money is plenty, have them install a higher stall speed lock-up converter.

One nice thing about the 700r4 is that you can adjust the shift firmness by taking slack out of the throttle valve.
Just make sure to mark it with some white paint so if you get lost [stuck in 1st gear] you have a reference point to get back to.

Consider adding a stand-alone trans cooler to this extreme usage off roader also !

I'm gonna murder this quote "Every 20 degrees F increase in transmission fluid temp, cuts the fluid life in half"
GM suggests heavy usage trans fluid & filter change every 30,000 miles.

Do NOT use el-cheapo transmission filters, get them from the Dealer, O'Reiley or other reputable dealers, please note I did NOT say Who sell's junk filters,,, those sometimes contain particles that can clog the valve body.

Hope this helps
Doctorvette